Syrian government troops have heavily bombarded Rastan, near Homs, in the centre of the country, killing at least 15 people, activists say.
More than 50 people have been killed in Rastan since a military operation there started at the weekend, reports say.
The offensive comes despite a government amnesty offer and the release of hundreds of detainees.
Opposition groups dismissed the moves and urged President Bashar al-Assad to resign and make way for democracy.
The groups, which comprise about 300 activists, are meeting in Antalya in neighbouring Turkey.
They said in a communique that Mr Assad should hand power to one of his two vice-presidents, without specifying which, and hold free elections within a year.
"The delegates have committed to the demands of the Syrian people to bring down the regime and support the people's revolution for freedom and dignity," the communique said.
The Local Co-ordinating Committee, which helps to organise and document the country's protests, gave the names of the people it said were killed in Rastan in the latest artillery and tank bombardments.
The committee said the offensive had hit at least two mosques and a bakery, as well as houses that collapsed, killing entire families.
Eyewitnesses told BBC Arabic that army and security forces are not able to take control the town, even though it has been surrounded by tanks over the past few days.
Following the announcement on Tuesday of a conditional amnesty, hundreds of detainees have been released.
More seem to be on the way, although it is not clear if the authorities intend to free all the 10,000 or more people believed to have been detained in the past 10 weeks and the thousands already in jail before that, says the BBC's Jim Muir in Beirut.
The authorities have announced the formation of a high-level commission to oversee a proposed national dialogue aimed at stabilising the situation.
More than 1,000 people have been killed in Syria since an uprising against President Assad began in March, activists say.
Reports from Syria are hard to verify independently, as foreign journalists are not allowed into the country.