Syrian rebels said they have seized control of the capital Damascus with reports suggesting that President Bashar al-Assad had boarded a plane and left the country for an unknown destination on Sunday.

Thousands in cars and on foot congregated at a main square in Damascus waving and chanting "Freedom", witnesses said.

"We celebrate with the Syrian people the news of freeing our prisoners and releasing their chains and announcing the end of the era of injustice in Sednaya prison," said the rebels.

Sednaya is a large military prison on the outskirts Damascus where the Syrian government detained thousands.

Just hours earlier, rebels announced they had gained full control of the key city of Homs after only a day of fighting, leaving Assad's 24-year rule dangling by a thread.

Intense sounds of shooting were heard in the centre of the Damascus, residents said on Sunday, although it was not immediately clear what the source of the shooting was.

In rural areas southwest of the capital, local youths and former rebels took advantage of the loss of authority to come to the streets in acts of defiance against Assad's rule.

Thousands of Homs residents poured onto the streets after the army withdrew from the central city, dancing and chanting "Assad is gone, Homs is free" and "Long live Syria and down with Bashar al-Assad".

Rebels fired into the air in celebration, and youths tore down posters of the Syrian president, whose territorial control has collapsed in a dizzying week-long retreat by the military.

The fall of Homs gives the insurgents control over Syria's strategic heartland and a key highway crossroads, severing Damascus from the coastal region that is the stronghold of Assad's Alawite sect and where his Russian allies have a naval base and air base.

Homs' capture is also a powerful symbol of the rebel movement's dramatic comeback in the 13-year-old conflict.

Swathes of Homs were destroyed by gruelling siege warfare between the rebels and the army years ago. The fighting ground down the insurgents, who were forced out.

Hayat Tahrir al-Sham commander Abu Mohammed al-Golani, the main rebel leader, called the capture of Homs a historic moment and urged fighters not to harm "those who drop their arms".

Rebels freed thousands of detainees from the city prison. Security forces left in haste after burning their documents.

Residents of numerous Damascus districts turned out to protest Assad on Saturday evening, and security forces were either unwilling or unable to clamp down.

Syrian rebel commander Hassan Abdul Ghani said in a statement early on Sunday that operations were ongoing to "completely liberate" the countryside around Damascus and rebel forces were looking toward the capital.

The Syrian army said it was reinforcing around Damascus, and state television reported on Saturday that Assad remained in the city. (Reuters)