A meeting of Syrian opposition politicians and rebels in Riyadh has produced a statement of principles to guide peace talks with the government.
The statement calls for a new "pluralistic regime that represents all sectors" in Syria, Reuters report.
It also stresses that President Bashar al-Assad and his aides could play no part in any transitional period.
There are conflicting reports as to whether the powerful rebel group Ahrar al-Sham signed the statement.
It had earlier complained that figures it regarded as too close to the government were being given too prominent a role, and pulled out.
But Reuters news agency said the group had signed the copy of the statement at the end of the two-day talks.
World powers want negotiations between a unified opposition delegation and President Bashar al-Assad's government on a political solution to the four-and-a-half-year conflict to start by 1 January.
Opposition groups had previously demanded Assad leave before any negotiations took place. But the statement says he would be allowed to stay until a transitional government was formed.
This signifies "a big change" on behalf of the opposition, Ibrahim Hamidi, the Syria editor of the Saudi owned al-Hayat newspaper, told the BBC.
Analysis by Jim Muir, BBC Middle East correspondent
What's important about the Riyadh meeting is that it creates a framework and mechanism for a broad spectrum of the Syrian opposition and rebels to engage in settlement negotiations with the regime.
That is what the Americans and others were looking for, as a prelude to another planned meeting of the outside powers - including Russia and Iran - later this month to prepare for rebel-regime negotiations in early January.
But that doesn't mean it's going to be plain sailing.
More than 250,000 people have been killed since an uprising against Mr Assad began in March 2011. Eleven million others have been forced from their homes.
The two-day conference in the Saudi capital brought together representatives of the main Western-backed political opposition alliance, the National Coalition, and of the National Co-ordination Committee, which is mostly tolerated by the authorities in Damascus although some of its members have been harassed and detained.
Most of the main rebel factions, including Ahrar al-Sham, also sent delegations.
Who are Ahrar al-Sham?
- An ultraconservative Islamist, or Salafist, rebel group that aims to topple Mr Assad and build an Islamic state
- Has vowed to achieve the latter through the ballot box and not force
- Part of Jaysh al-Fatah, an alliance that includes the al-Qaeda-affiliated al-Nusra Front
- Earlier on Thursday, said it had withdrawn from the Riyadh conference - and is now reported to have signed the deal
- Had disputed the "main role" accorded to "personalities linked to the regime" - an apparent reference to members of the National Co-ordination Committee - and the inadequate representation of "revolutionary factions"
Reuters quoted a joint statement as saying that delegates had backed a "democratic mechanism through a pluralistic regime that represents all sectors of the Syrian people". It would include women and would not discriminate on religious, sectarian or ethic grounds, the statement added.
Delegates also committed to preserving state institutions and restructuring the security forces.
Mr Assad's staunch ally Russia - which launched an air campaign in September to prop up his government - supports the implementation of the 2012 Geneva Communique, which calls for the establishment of a transitional administration.
But the president has said his departure is out of the question before elections are held, and recently warned that peace talks could not begin while the country was occupied by "terrorists" - a term he uses to describe all armed opponents.
US Secretary of State John Kerry meanwhile said that the new International Syrian Support Group - which includes the Arab League, the EU, the UN and 17 countries - was "working toward the potential of a meeting in New York" on 18 December.
The Syrian Kurds, who control large parts of northern Syria, were not invited to Riyadh. They held their own conference, at the same time, on the future of Syria.