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.
Syria’s deputy foreign minister later insisted Mr. Assad would not step aside.
Faisal Mekdad told the BBC that demands that the country’s elected leader be removed were “crazy” and “unbelievable”.
“We are saying, let the Syrian people decide their own fate. Their own future without intervention. Don’t make preconditions who will rule Syria,” he said.
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.
If the document on a transition in Syria had been produced by the political leaders of a victorious army at the gates of Damascus, it could be seen as an enlightened blueprint for the future.
But the regime in Damascus will dismiss it, because President Bashar al-Assad seems more secure in the capital than at any other time since the war started.
In 2013, when the chemical attack on the city’s rebel-held eastern suburbs seemed certain to provoke American air strikes the regime looked shaky and afraid. But the strikes never came, and since then the president and his men have strengthened their positions. Crucially, Russia has intervened directly on the battlefield. President Assad cannot win as things stand, but he cannot lose either.
The war gets more complicated every year. Regional superpowers and the world’s most powerful countries have intervened. Dozens of different and sometimes overlapping rebel groups operate on the battlefields. All have their own agendas. It’s a mini world war. No wonder diplomacy fails. -BBC