Assad must go after six months of talks, HNC says, adding it will reject any US-Russia deal that strays from its terms.
Syria’s main opposition bloc has put forward a plan for a political transition and a ceasefire to end more than five years of civil war in the country.
The High Negotiations Committee (HNC) said on Wednesday that the proposed process would start with six months of negotiations aimed at setting up a transitional administration made up of figures from the opposition, the government and civil society.
President Bashar al-Assad would be required to leave office at the end of those six months, the bloc, which represents Syria’s political and armed opposition factions, said at a meeting in London attended by foreign ministers from around the world.
The transitional body would then run the country for 18 months, after which there would be elections.
Chief opposition negotiator Riyad Hijab, who defected from the government in 2012 after being appointed prime minister, said the HNC would reject any agreement struck by Russia and the US if it largely differed from the HNC’s terms.
"If what the Russians and the Americans agree upon is very much different from what the Syrians aspire to, then we shall not accept it," Hijab said.
Reporting from the meeting in the UK capital, Al Jazeera’s diplomatic editor James Bays said the plan was "the most detailed blueprint that the opposition has come up with of the way they see things going forward".
Our correspondent noted, however, that it was unlikely that a transition based on the HNC’s proposal would actually take place, as there was "no prospect of any negotiations" other than the talks in Geneva between the US and Russia.
In response to a question by Bays as to how the HNC will "restart the diplomatic process", Hijab said: "We have gone through rounds of talks and a political process in 2014 and unfortunately we failed.
"The political process failed because there was a refusal to talk about the political transition by [UN Special Envoy to Syria] Staffan de Mistura. He knows that over the past few months the regime has refused to talk about a political transition and, practically, we do not have any negotiating rounds happening in Geneva because the regime was very rigid and absent.
"We feel we have to move to a new phase, and the new phase cannot happen without a political transition and the political seriousness that will compel the regime and its allies."
Moscow and Washington, which have been involved in ceasefire negotiations in recent days, are backing opposite sides in the Syrian conflict.
US Secretary of State John Kerry was planning to fly to Geneva to hold talks with the Russian Foreign Minister Sergey Lavrov on Syria on Thursday, sources at the London meeting told Al Jazeera.
Kerry, who called in at the meeting, said there is a "new map being drawn up between the US and Russia" that would require the Syrian government not to fly over territory controlled by moderate rebels; areas where there are civilians; and regions with the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) and Jabhat Fateh al-Sham fighters, Bays reported.
"In effect then, a no-fly zone in those areas," our correspondent added.
"[Kerry] also said there will be a new and enhanced joint operations centre with the US and the Russians to control all of this," but added that he could not guarantee that "the Syrian regime and Russia will actually stick to an agreement, if an agreement is signed", according to Bays.
Earlier on Wednesday, Adel al-Jubeir, the foreign minister of Saudi Arabia, which backs the HNC, said that he believed a deal between the US and Russia could be close.
"There is a possibility of arriving at an understanding over the next 24 hours or so," he said.
"Then we will test the seriousness of Assad and his allies in terms of complying to a ceasefire like this."
In a statement on Wednesday, the UK government, which hosted the London meeting, said that Assad was "directly responsible for the crisis in Syria" and called for a political transition.
The Syrian conflict began as a mostly unarmed uprising against Assad in March 2011, but quickly escalated into a full-blown civil war.
More than 280,000 Syrians have been killed throughout the five years of bloodshed, 4.8 million have fled the country, and 6.6 million have become internally displaced by the violence, according to the UN.