Syria's main opposition negotiating group, the High Negotiations Committee, has presented its road map to a new political settlement at a conference in London.
The proposed process would start with six months of negotiations to set up a transitional administration in Syria.
That administration would be made up of figures from the opposition, the government and civil society.
It would require President Bashar al-Assad to leave office at the end of those six months.
The group's general coordinator, Riyad Hijab, says the transitional body would then run the country for 18 months.
After that, he says, elections would be held.
"We feel that we have to move to a new phase. This new phase cannot take place except through political transition and with a serious political will that will compel the regime and its allies."
The conference host, British foreign secretary Boris Johnson, has backed the plan, which he says could help stalled United Nations-mediated peace talks restart in Geneva.
"It is a plan, a vision and a plan, for Syria that is democratic, that is pluralistic, that will take that country forward."
French foreign minister Jean-Marc Ayrault has commended the plan.
"The Syrian Council, presided over by Mr Hijab, who attended this meeting, has done remarkable work in its proposition for a united, democratic and secular Syria which would at the same time allow respect for all religions and all minorities."
But Syria's deputy foreign minister, Faisal Mekdad, has rejected the proposal that President Assad step down.
Mr Mekdad says the president was democratically elected.
"No-one has the right to impose on the Syrian people who will lead them. For the last five years, they have been demanding the removal of a Syrian-elected president, which is crazy, which is unbelievable."
The High Negotiations Committee says it will reject any deal on Syria's fate struck by Russia and the United States that is very different from its own proposed transition plan.
Russia and the United States are backing opposite sides in the five-and-a-half-year-old conflict.
The Russians are fighting on Mr Assad's side, while the United States backs the opposition groups and insists Mr Assad must go.
The two powers have been negotiating in recent days but have failed so far to reach an agreement.
US defence secretary Ash Carter says talks cannot go on forever.
"At the end of the day, it's going to depend upon whether Russia does what it said it was going to do when it came into Syria, which is fight terrorism and support a political transition that brings an end to the Syrian civil war."
A member of the High Negotiations Committee's Women's Consultative Committee, Hind Kabawat, says the international community must end the bloodshed and restore peace.
"As a mother, I cannot any more tolerate killing the Syrian civilians. We need to stop the killings. And this is also the commitment and the responsibility of the international community, to work and help us in preventing death to our children."
