Qatari prime minister Hamad bin Jassim al-Thani told a news conference in Cairo that the plan called for Mr Assad to hand power to a national unity government under a compromise president within two months and for parliamentary and presidential elections to be held within six months.

It is a dramatic response to a 10-month conflict that has already killed 5,000 Syrians, most of them victims of the security forces, and shows no signs of abating.

“We’re going to the security council,” the Qatari premier said, sitting alongside Arab League secretary general Nabil Arabi.

“We know what the Syrian people are going through. We are doing as much as we can to help.”

There was no official Syrian reaction to the proposal, which bears similarities to the Arab League plan meant to usher out of power Yemeni president Ali Abdullah Saleh. Mr Thani told reporters that the plan received widespread Arab support.

However, Syria’s state news agency quoted an anonymous government official as saying that Damascus condemned the plan as “a violation of its sovereignty and flagrant interference in its internal affairs”.

It said the Arab League proposal was a further evolution of the “conspiracy Syria is being subjected to”, echoing the regime’s narrative that it is the victim of terrorists and armed gangs backed by foreign powers.

The Arab League plan came as a shock to some of the group’s monitors in Damascus, who have been in a state of uncertainty for days about the future of their mission after its mandate expired last week.

One monitor said the Arab League position was much tougher than he had expected, adding that he – like other observers – had not received any directions from his government yet on whether he should stay or go.

Mr Assad recently condemned the Arab League and other Arab states as dupes of western powers. It remains unlikely that he and his entourage would peacefully hand over the power and wealth they have accumulated in more than four decades of one-family, single-party rule.

The Arab League announcement capped a day of question marks about the future of an unprecedented Arab League mission in Syria.

Mr Nabil said that the month-long monitoring operation would continue, despite an announcement by Saudi Arabia that it would withdraw because it served as a cover for the Damascus regime.

“My country is going to withdraw its observers,” Saud el Faisal, Saudi foreign minister, said in a statement, referring to the mission.

“We are not going to accept being used as witnesses to crimes or to hide their crimes.

“We are calling on the international community to bear its responsibility and that includes our brothers in Islamic states and our friends in Russia, China, Europe and the United States,” Prince Saud said.

He called for “all possible pressure” to push Syria to adhere to the Arab peace plan.

His comments came after Mr Nabil said in a statement that the mission should be extended by a month and that the number of observers should be increased.

Mr Nabil defended the extension, saying the report by the mission’s controversial head, Sudanese general Mostafa Ahmad al-Dabi, showed there was “progress” in Syria, but he acknowledged that “the Syrian government until now hasn’t done what it has promised”.

Mr Nabil also accused Syria of failing to abide by a deal it signed a month ago to end the violent suppression of the 10-month anti-government uprising in the country, adding that the regime had tried to manipulate the observers to suit its own purposes.

But “the mission was aware of that strategy and resisted it,”, he said.

Opposition activists continued to call for an end to the Arab League mission and demanded that the UN Security Council place sanctions on Syria.

The mission has enraged Syrian opposition activists, who accuse the Arab League of providing diplomatic cover for a brutal political crackdown.

“However much they increase the number of observers it’s never going to be enough,” said Amer Al-Sadeq, a member of the Syrian revolution coordinators union, an opposition group based outside the country.

The local coordination committees, a network of activists inside and outside Syria, alleged in an announcement on Sunday that nearly 1,000 Syrians had died at the hands of security forces since the Arab League mission began, more than a third of them in the city of Homs.

Inspired by the Arab Spring revolutions shaking the Middle East and north Africa, the Syrian uprising began 10 months ago when Syrian security forces suppressed protests against the arrest and torture of a group of alleged teen vandals in city of Dera’a.

The UN estimates that more than 5,000 Syrians have died in the unrest.

Mr Assad’s opponents remain divided. The umbrella Syrian national council, which demands radical regime change, is at odds with the national coordination committee for democratic change, which opposes foreign military intervention and is open to dialogue with Mr Assad.

Meanwhile, activists advocating armed struggle are eclipsing those who insist on peaceful resistance, while Syrian Kurds remain wary of Arab nationalists who in turn mistrust the Islamists.

“We’re all over the place,” acknowledged Mohammad Obazi, an opposition activist in Cairo.

“We, the foreign opposition, most of us are exiles. But the opposition leaders that are inside Syria are from the regime. Most of the opposition doesn’t agree on one thing because they all have their different ideologies.”

As a protest march got under way from the Syrian embassy to the Arab League on Sunday, anti-regime activists scuffled violently with each other. Some wanted to storm the mission, while others tried to hold them back.

But Mr Sadeq predicted neither the opposition’s disarray nor the monitors’ actions would affect dynamics on the ground.

“They’re going to keep demonstrating,” he said. “There’s no relation between the [Arab League] report and our project to overthrow the regime.”

Additional reporting by Dena Elsisy, and Michael Peel and Abigail Fielding-Smith in Damascus