The new grouping, called the Union of Syrian Democrats, includes heavyweight activists such as Michel Kilo, a Christian writer and human rights activist.
It appears to be an attempt to counterbalance the influence of the Syrian Muslim Brotherhood inside the National Coalition, the country's main opposition bloc.
The 250 "members of the group, who come from different political streams, are united on the principal of democracy," one of its members, Kamal Labwani, told AFP.
"If the Muslim Brotherhood are opposed to the construction of a civil state, this group is directed against them. If they support this project, they are our allies," he said.
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"Our problem with the Muslim Brotherhood is that they say one thing and do another... They say they want a civil state, but in practice, they don't."
The Muslim Brotherhood is the best organised of the groups arrayed in opposition to President Assad's regime.
But it has been criticised by activists for trying to dominate the opposition, boosted by the support of Qatar.
The new opposition group, launched in Cairo, will hold a series of meetings in coming months to discuss their structure.