The broad-based High Negotiations Committee (HNC) proposed a six-month negotiating phase between the regime and the opposition.
The subsequent 18 months would see Syria governed by a transitional body, made up of opposition figures, current government representatives and members of civil society, according to a 25-page blueprint.
"Syria wants to see Bashar leave. If Bashar leaves will the fighting go on? No," HNC head Riad Hijab said.
The proposals, and the talks hosted by British Foreign Secretary Boris Johnson, come after intense discussions by the United States and Russia over a possible path to end the five and a half year conflict.
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US Secretary of State John Kerry will attend the talks in London via videolink, but Russia is not represented.
"The aim of the meeting in London is to prepare a common position and make the case to the United States, while there are persistent rumours about the conclusion of a US-Russian deal," a French diplomatic source told AFP.
The HNC's plans are largely in line with existing international proposals for a post-war Syria, although unlike the so-called Vienna framework, they are clear about the president's future.
The goal is "building a political system that protects freedoms, safeguards individual rights, and that is founded upon the principles of liberty, equality, citizenship, and justice", it said.
In an article in today's edition of The Times, Johnson urged Moscow to cease support for the Syrian president.
The British diplomat accused Assad of "barbaric military tactics" in the ongoing conflict and criticised Russia's "seemingly indefensible conduct" in backing him.
"The entire international community is committed, at least in principle, to getting rid of the Syrian dictator. Even the Russians have accepted that there must be political transition," he wrote.