The surprise attack, led by the Islamist group Hayat Tahrir al-Sham, was the boldest rebel assault for years in a civil war where frontlines had largely been frozen since 2020.
The war, which has killed hundreds of thousands of people and displaced many millions, has ground on since 2011 with no formal end, although most major fighting halted years ago after Iran and Russia helped Assad's government win control of most land and all major cities.
Hayat Tahrir al-Sham, or HTS, once known as al-Nusra Front, is designated a terrorist group by the United States, Russia, Turkey and other countries.
Aleppo had been firmly held by the government since a 2016 victory there, one of the war's major turning points, when Russian-backed Syrian forces besieged and lay waste to rebel-held eastern areas of what had been the country's largest city.
"I am a son of Aleppo, and was displaced from it eight years ago, in 2016. Thank God we just returned. It is an indescribable feeling," said Ali Jumaa, a rebel fighter, in television footage filmed inside the city.
Acknowledging the rebel advance, the Syrian army command said insurgents had entered large parts of Aleppo.
After the army said it was preparing a counterattack, air strikes targeted rebel gatherings and convoys in the city, the pro-government newspaper al-Watan reported.
One strike caused casualties in Aleppo's Basel square, a resident told Reuters.
Overnight, images from Aleppo showed a group of rebel fighters gathered in the city's Saadallah al-Jabiri Square, a billboard of Assad looming behind them.
Rebel fighters damaged depictions of Syrian President Bashar Assad after entering Aleppo. (AP PHOTO)
Images filmed on Saturday showed people posing for photos on a toppled statue of Bassil al-Assad, late brother of the president.
Fighters zipped around the city in flatback trucks and milled around in the streets.
A man waved a Syrian opposition flag as he stood near Aleppo's historical citadel.
The Syrian military command said militants had attacked in large numbers and from multiple directions, prompting "our armed forces to carry out a redeployment operation aimed at strengthening the defence lines in order to absorb the attack, preserve the lives of civilians and soldiers".
The rebels also took control of Aleppo airport, according to a statement by their operations room and a security source.
Two rebel sources also said the insurgents had captured the city of Maraat al Numan in Idlib province, bringing all of that area under their control.
The fighting revives the long-simmering Syrian conflict as the wider region is roiled by wars in the Gaza Strip and Lebanon, where a truce between Israel and the Iran-backed Lebanese group Hezbollah took effect on Wednesday.
With Assad backed by Russia and Iran, and Turkey supporting some of the rebels in the northwest where it maintains troops, the offensive has brought into focus the conflict's knotted geopolitics.
Fighting in the northwest had largely abated since Turkey and Russia reached a de-escalation agreement in 2020.
Russian Foreign Minister Sergei Lavrov held a phone call with his Turkish counterpart Hakan Fidan, discussing the situation in Syria, the Russian foreign ministry said on Saturday.
"Both sides expressed serious concerns at the dangerous development of the situation," the ministry said.
They agreed that it was necessary to co-ordinate joint actions to stabilise the situation in the country.
Turkish security officials had said on Thursday that the country had prevented operations which opposition groups wanted to organise, in order to avoid further tensions in the region.
Iranian Foreign Minister Abbas Araqchi told Lavrov in a phone call that the rebel attacks were part of an Israeli-US plan to destabilise the region, Iranian state media said.
The Syrian Civil Defence, a rescue service operating in opposition-held parts of Syria, said in a post on X that Syrian government and Russian aircraft carried out air strikes on residential neighbourhoods in rebel-held Idlib, killing four civilians and wounding six others.
Mustafa Abdul Jaber, a commander in the Jaish al-Izza rebel brigade, said the rebels' speedy advance had been helped by a lack of Iran-backed manpower to support the government in the broader Aleppo province.
Iran's allies in the region have suffered a series of blows at the hands of Israel as the Gaza war has expanded through the Middle East.
