Image copyright Reuters The umbrella group representing Syria's political and armed opposition factions has set out a plan for a political transition to end five years of war.
The High Negotiations Committee (HNC) proposed holding six months of negotiations with President Bashar al-Assad, accompanied by a full ceasefire.
Mr Assad would then hand over power to a unity government that would run Syria for 18 months and organise elections.
Saudi Arabia, which backs the HNC, said the plan would test Mr Assad's allies.
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But the Gulf kingdom's foreign minister, Adel al-Jubeir, told the BBC that he was not optimistic that Russia and Iran were prepared to put the "necessary pressure" on the president "in order to comply with the will of the international community".
Mr Assad has consistently refused to step down since the beginning of the war, which has left more than 250,000 people dead, and correspondents say the recent battlefield successes of his forces have only hardened that view.
The HNC's "Vision for Syria" was unveiled in London by its general co-ordinator, Riyad Hijab, who defected in 2012 while serving as prime minister.
He said the plan envisaged three phases:
- Six months of negotiations between opposition and government representatives, using a 2012 document known as the Geneva Communique as a basis for discussions. Both sides would also commit to a temporary truce, lift sieges, allow full humanitarian access and release detainees
- A transitional period lasting a year and a half would start with the establishment of a transitional governing body with full executive powers, as well as the departure of Mr Assad and other senior officials accused of committing "heinous crimes". A new constitution would be drafted during this phase, and a democratic and pluralistic political system established
- Changes to the constitution would be agreed, the outcomes of a national dialogue implemented, and elections held under UN supervision
Later, foreign ministers from the Friend of Syria group of countries that support the opposition will discuss the HNC's proposals.
UK Foreign Secretary Boris Johnson, who is hosting the meeting, wrote in The Times that there "was still a chance that this vision can be made to work".
Image copyright AP"If the Russians and Americans can together create a ceasefire, then the talks can restart in Geneva with the difference, perhaps, that all sides will by then have seen at least the scaffolding of a post-Assad Syria," he added.
Moscow and Washington, which backs the opposition and insists Mr Assad must go, have so far failed to reach an agreement on a new cessation of hostilities.
After a meeting with President Vladimir Putin at the G20 summit in China on Monday, Barack Obama said they had been unable to close "gaps of trust".
US officials are particularly concerned by the deteriorating conditions in and around Syria's second city, Aleppo, where rebel-held eastern areas are once again under siege after government forces advanced over the weekend.
On Monday, more than a 100 people were admitted to hospital following a suspected chlorine gas attack on the rebel-held Sukkari district. Activists in the city said one person died after bombs were dropped from government helicopters.