BEIRUT (AP) – The Syrian government fell early Sunday in a stunning end to the 50-year rule of the Assad family after a sudden rebel offensive sprinted across government-held territory and entered the capital in 10 days.

Syrian state television aired a video statement by a group of men saying that President Bashar Assad had been overthrown and all detainees in jails had been set free.

The man who read the statement said the Operations Room to Conquer Damascus, an opposition group, called on all opposition fighters and citizens to preserve state institutions of “the free Syrian state.”

The statement emerged hours after the head of a Syrian opposition war monitor said Mr. Assad had left the country for an undisclosed location, fleeing ahead of insurgents who said they had entered Damascus following the remarkably swift advance across the country. The ousted Syrian leader had fled to Moscow and received asylum from his longtime ally, Russian media said later on Sunday.

Syrian Prime Minister Mohammed Ghazi Jalali said the government was ready to “extend its hand” to the opposition and turn its functions over to a transitional government.

“I am in my house and I have not left, and this is because of my belonging to this country,” Mr. Jalili said in a video statement. He said he would go to his office to continue work in the morning and called on Syrian citizens not to deface public property.

As daylight broke over Damascus, crowds gathered to pray in the city’s mosques and to celebrate in the squares, chanting “God is great.” People also chanted anti-Assad slogans and honked car horns. In some areas, celebratory gunshots rang out.

Soldiers and police officers left their posts and fled, and looters broke into the headquarters of the Ministry of Defense.

“My feelings are indescribable,” said Omar Daher, a 29-year-old lawyer. “After the fear that he [Mr. Assad] and his father made us live in for many years, and the panic and state of terror that I was living in, I can’t believe it.”

Mr. Daher said his father was killed by security forces and his brother was in detention, his fate unknown.

“Damn his soul and the soul of the entire Assad family,” said Ghazal al-Sharif, another reveler in central Damascus. “It is the prayer of every oppressed person and God answered it today. We thought we would never see it, but thank God, we saw it.”

The police headquarters in the capital appeared to be abandoned, its door left ajar with no officers outside. An Associated Press journalist shot footage of an abandoned army checkpoint where uniforms were discarded on the ground under a poster of Mr. Assad’s face. Footage broadcast on opposition-linked media showed a tank in one of the capital’s central squares.

It was the first time opposition forces had reached Damascus since 2018, when Syrian troops recaptured areas on the outskirts of the capital following a yearslong siege.

The pro-government Sham FM radio reported that the Damascus airport had been evacuated and all flights halted.

The insurgents also announced they had entered the notorious Saydnaya military prison north of the capital and “liberated” their prisoners there.

The night before, opposition forces took the central city of Homs, Syria’s third largest, as government forces abandoned it. The city stands at an important intersection between Damascus, the capital, and Syria’s coastal provinces of Latakia and Tartus—the Syrian leader’s base of support and home to a Russian strategic naval base.

The rebels had already seized the cities of Aleppo and Hama, as well as large parts of the south, in a lightning offensive that began November 27. Analysts said rebel control of Homs would be a game-changer.

The rebels’ moves into Damascus came after the Syrian army withdrew from much of the southern part of the country, leaving more areas, including several provincial capitals, under the control of opposition fighters.

The advances in the past week were by far the largest in recent years by opposition factions, led by a group that has its origins in al-Qaida and is considered a terrorist organization by the U.S. and the United Nations. In their push to overthrow Mr. Assad’s government, the insurgents, led by the Hayat Tahrir al-Sham group, or HTS, met little resistance from the Syrian army.

In Damascus, people rushed to stock up on supplies. Thousands went to Syria’s border with Lebanon, trying to leave the country. Lebanese border officials closed the main Masnaa border crossing late Saturday, leaving many stuck waiting.

Many shops in the capital were shuttered, a resident told The Associated Press, and those still open ran out of staples such as sugar. Some were selling items at three times the normal price.

The UN said it was moving noncritical staff outside the country as a precaution.

The Insurgents’ March

A commander with the insurgents, Hassan Abdul-Ghani, posted on the Telegram messaging app that opposition forces had begun the “final stage” of their offensive by encircling Damascus.

HTS controls much of northwest Syria and in 2017 set up a “salvation government” to run day-to-day affairs in the region. In recent years, HTS leader Abu Mohammed al-Golani has sought to remake the group’s image, cutting ties with al-Qaida, ditching hard-line officials and vowing to embrace pluralism and religious tolerance.

The Syrian government has referred to opposition gunmen as terrorists since conflict broke out in March 2011.

Qatar’s top diplomat, Sheikh Mohammed bin Abdulrahman Al Thani, criticized Mr. Assad for failing to take advantage of the lull in fighting in recent years to address the country’s underlying problems. “Assad didn’t seize this opportunity to start engaging and restoring his relationship with his people,” he said.

Joy and Fear

For the first time in 50 years, the question of how Syria will be governed is wide open. The end of the Assad family’s rule is for many Syrians a moment of mixed joy and fear, of the total unknown.

The insurgency that swept Mr. Assad out of power is rooted in Islamist jihadi fighters. Its leader says he has renounced past ties to al-Qaida, and he has gone out of his way to assert a vision of creating a pluralistic Syria governed by civil institutions—not dictators and not ideology.

But even if he is sincere, he is not the only player. The insurgency is made up of multiple factions, and the country is riven among armed groups, including U.S.-backed Kurdish fighters controlling the east. Remnants of the old regime’s military—and its feared security and intelligence services—could coalesce once again.

Foreign powers with their own interests have their hands deep in the country, and any of them—Russia, Iran, Turkey, the United States and Israel—could act as spoilers.

Syria’s multifaith and multiethnic population sees itself poised on a moment that could tip either into chaos or cohesion. The country’s Sunni Muslims, Shiite Alawites, Christians and ethnic Kurds have often been pitted against each, whether by Mr. Assad’s rule or a 14-year civil war.

Divisions from the conflict run deep, and many worry about revenge killings, whether against former figures of Assad’s state or—more frightening—whole communities seen as backing the old system.

The civil war displaced half of Syria’s prewar population of 23 million. Many who fled are watching developments closely to determine whether the time has come to return.

Right now there are only questions.

How Will Syria Be Governed?

In the short period following Mr. Assad’s abrupt fall, rebel leader Ahmad al-Sharaa, formerly known as Abu Mohammed al-Golani, has sought to reassure Syrians that the group he leads—Hayat Tahrir al-Sham, or HTS – does not seek to dominate the country and will continue government services. He has spoken of setting up a decentralized governance system.

Government officials who remained in Damascus as Mr. Assad fled—including Prime Minister Jalali—have met with the rebels to discuss the transfer of power.

The Al Jazeera television network reported Monday that HTS had decided to appoint the head of the “salvation government” running its stronghold in northwest Syria, Mohammed Al-Bashir, to form a transitional government. There was no official confirmation.

Details on what form the government will take have been scarce.

The rebels likely did not expect to be saddled with running an entire country when they launched their offensive against Aleppo less than two weeks ago, said Qutaiba Idlbi, a senior fellow at the Atlantic Council’s Rafik Hariri Center and Middle East Programs. The rapid fall of Damascus and the melting away of police and military, left security challenges, he said.

The only existing framework for a transition is no longer relevant. UN Security Council Resolution 2254 had called for a political process involving both Mr. Assad’s government and opposition groups.

“Everyone’s saying, especially rebels on the ground, ‘That framework is no longer applicable, because there is no longer a regime. We’re not going to give the regime in politics what they lost through military means,’” Mr. Idlbi said.

So far, public sector workers have not heeded calls from the caretaker prime minister to go back to their jobs—causing troubles in places like airports, borders and at the Foreign Ministry, said Adam Abdelmoula, the UN’s humanitarian coordinator for Syria.

“I think it will take a couple of days—and a lot of assurance on the part of the armed groups—for these people to return to work again,” he said. In the current chaos, UN workers have had difficulty accessing the country, and that has hampered distribution of humanitarian aid, he said.

How Inclusive Will the Insurgents Be?

The insurgents have sought to reassure Syria’s religious minorities that they will not be targeted, despite HTS’ fundamentalist Sunni Muslim origins.

So far the civil peace seems to be holding. The insurgents have appeared disciplined, working to keep order, with no sign of reprisals. Experts say only time will tell what post-Assad Syria will look like.

“Everyone’s still willing to really engage, really work with others,” said Haid Haid, a consulting fellow at the Middle East and North Africa program of Chatham House. “That sort of positive atmosphere is crucial, but it might not last long.”

Splits could open as decisions are made.

It cannot be guaranteed all the fighters within the HTS will back Mr. al-Sharaa’s talk of a pluralist system. Outside Damascus’ historic Hamadiyeh market on Sunday, around a dozen fighters chanted, “Down, down with a secular state”—a sign that at least some among the insurgents may seek a harder Islamist line.

“The opposition is not a homogenous movement,” said Burcu Ozcelik, a senior research fellow for Middle East Security at the Royal United Services Institute think tank in London.

There are multiple armed opposition groups, including forces in the south who are distinct from HTS and the Turkish-backed groups in the north. Internal fractures within the HTS-led movement, “which may become more salient in the weeks and months to come, may lead to discord and threaten Syrian stability,” Mr. Ozcelik said.

There may be pressure to purge former members of Mr. Assad’s large state bureaucracy, especially those employed as part of a vast security state that included informers and officers widely hated for torture, abuses and corruption.

Insurgents and many in the public do not want them to return. But a purge can spark a destabilizing backlash—as when U.S. administrators disbanded Iraq’s army after Saddam Hussein’s fall in 2003, fueling a Sunni insurgency.

Syria’s Alawite population is feeling particularly vulnerable. Mr. Assad and his family were Alawites—a branch of Shia Islam—and many among the Sunni insurgents see the community as his loyalists.

The Question of the Kurds Looms Large

Kurdish-led forces allied with the United States have run a semi-autonomous zone in Syria’s northeast for years, where they have been a key player in the fight against the Islamic State militant group. While both were opponents of the government during the civil war, the relationship between the Kurds and the Arab opposition groups is tense.

HTS has been extending an olive branch to the Kurds. Reintegrating the east would likely mean some form of concession to Kurdish autonomy.

But that risks angering neighboring Turkey, which vehemently opposes the Kurdish factions that run Syria’s east. Already, Turkish-backed insurgents allied with HTS have taken the opportunity to push the Kurds out of some pockets of territory, seizing the northern town of Manbij, and clashes have broken out in other areas.

While the insurgents’ largely benign approach to minorities so far has allayed many international worries, Mr. Abdelmoula said, “those pockets of fighting are very significant because the fighting is mostly along ethnic lines. And that’s dangerous.”