Activists say democratic transition will safeguard the country from "a period of violence, chaos and civil war".
"Syrian opposition figures have said their "massive grassroots revolution" will break the regime unless Bashar al-Assad, the president, leads a transition to democracy.
The statement on Wednesday from an umbrella group of opposition activists in Syria and abroad, called the National Initiative for Change, said a democratic transition will "safeguard the nation from falling into a period of violence, chaos and civil war."
"If the Syrian president does not wish to be recorded in history as a leader of this transition period, there is no alternative left for Syrians except to move forward along the same path as did the Tunisians, Egyptians and Libyans before them," the statement said.
The opposition in Syria is getting more organised as anti-government protests gain strength, but it is still fragmented.
Meanwhile, witnesses said a convoy of about 30 tanks were seen on the Damascus circular highway outside the capital.
Witnesses also said troops had poured into the Damascus suburb of Douma overnight and were also deployed around the coastal city of Baniyas....."