Russian news agencies late Sunday said Assad and his family were in Moscow.

Crowds toured Assad's luxurious home after the rebels declared he had fled, a spectacular end to five decades of brutal Baath party government.

The government fell 11 days after the rebels began a surprise advance more than 13 years after Assad's crackdown on anti-government protests ignited Syria's civil war, which had become largely dormant until the rebel push.

"This victory, my brothers, is historic for the region," Abu Mohammed al-Jolani, leader of the Islamist Hayat Tahrir al-Sham group (HTS) that spearheaded the advance, said in an address at the landmark Umayyad Mosque in Damascus.

US President Joe Biden said Assad should be "held accountable" but called the nation's political upheaval a "historic opportunity" for Syrians to rebuild their country.

"The fall of the regime is a fundamental act of justice," Biden said from the White House.

'Syria is ours'

Residents cheered in the streets as the rebel factions heralded the departure of "tyrant" Assad, saying: "We declare the city of Damascus free."

Celebratory gunfire sounded along with shouts of, "Syria is ours and not the Assad family's".

AFP correspondents saw dozens of men, women and children wandering through Assad's modern, spacious home whose rooms had been stripped bare.

"I can't believe I'm living this moment," tearful Damascus resident Amer Batha told AFP by phone.

"We've been waiting a long time for this day," he said.

The rebel factions on Telegram proclaimed the end to "50 years of oppression under Baath rule, and 13 years of crimes and tyranny and displacement".

It is, they said, "the start of a new era for Syria."

The foreign ministry of Assad's key backer, Russia, had announced earlier Sunday that Assad had resigned from the presidency and left Syria.

The head of war monitor the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP: "Assad left Syria via Damascus international airport before the army security forces left" the facility.

Later Sunday, a Kremlin source told Russian news agencies that he and his family had arrived in Moscow where they had been granted asylum "on humanitarian grounds".

'Historic opportunity'

Around the country, people toppled statues of Hafez al-Assad, Bashar al-Assad's father and the founder of the repressive system of government he inherited.

For the past 50 years in Syria, even the slightest suspicion of dissent could land one in prison or get one killed.

During their advance, the rebels said they had freed prisoners, including on Sunday at the Sednaya facility, notorious for the darkest abuses of Assad's era.

UN war crimes investigators urged those taking charge in the country to ensure the "atrocities" committed under Assad's rule are not repeated.

Amnesty International called this a "historic opportunity" for those responsible for the abuses in Syria to face justice.

The end of Assad's rule came just hours after HTS said it had captured the strategic city of Homs.

Homs was the third major city seized by the rebels, who began their advance on November 27, the same day a ceasefire took place in neighboring Lebanon between Israel and the Iran-backed Hezbollah movement.

Hezbollah had supported Assad during the long civil war but has been severely weakened by Israeli strikes.

The group's forces "vacated their positions around Damascus", a source close to the group said Sunday.

HTS is rooted in the Syrian branch of Al-Qaeda but has sought to soften its image in recent years. It remains listed as a terrorist organization by Western governments.

On Sunday afternoon the rebels announced a curfew in the capital until 5:00 a.m. Monday.

The commander of Syria's US-backed, Kurdish-led Syrian Democratic Forces (SDF), which controls much of northeast Syria, hailed the fall of Assad's "authoritarian regime" as "historic".

A military council affiliated with the SDF clashed Sunday with Turkish-backed Syrian fighters in Syria's north, leaving 26 fighters from both sides dead, the Observatory said, as the Turkish-backed group launched an offensive on the Manbij area. 

'We're going home'

The Observatory said Israel had struck government security buildings and weapons depots Sunday on the outskirts of Damascus, as well as in the eastern Deir Ezzor province.

Israeli Prime Minister Benjamin Netanyahu said the overthrow of Assad was a "historic day in the... Middle East" and the fall of a "central link in Iran's axis of evil".

"This is a direct result of the blows we have inflicted on Iran and Hezbollah, Assad's main supporters," he added.

The UN envoy for Syria said the country was at "a watershed moment". Turkey, which has historically backed the opposition, called for a "smooth transition".

Iran said it expected "friendly" ties with Syria to continue, even as its embassy in Damascus was vandalized.

Since the start of the rebel offensive, at least 910 people, mostly combatants but also including 138 civilians, have been killed, the Observatory said.

Syria's war has killed more than 500,000 people, and forced half of the population to flee their homes.

Millions fled abroad.

"I can barely remember Syria," said Reda al-Khedr, who was only five years old when he and his mother escaped Syria's Homs in 2014.

"But now we're going to go home to a liberated Syria," he told AFP in Cairo.

Liberated, but facing enormous challenges.

European Commission President Ursula von der Leyen said Sunday the bloc would help rebuild a Syria that safeguards minorities after Assad's fall.