The conference comes one day after hundreds of thousands of Syrians mounted the largest protests since the country's uprising began more than four months ago. Syrian security forces fired on the crowds, killing at least 28 people.
On Saturday, some 400 people took part in the "National Salvation Conference" in Istanbul to form a unified opposition to Assad, whose family has ruled Syria for more than 40 years.
"The regime had kidnapped the entire state, and we want it back," said Haitham al-Maleh, who headed the conference. The 80-year-old lawyer who spent years in Syrian prisons for his political activism recently left Syria, fearing for his life, and is now in İstanbul.
"This regime cannot rob us of our freedom," al-Maleh said.
Activists say the regime's crackdown on dissent has killed some 1,600 people, most of them unarmed protesters, since March. But the government disputes the toll and blames a foreign conspiracy for the unrest, saying religious extremists - not true reform-seekers - are behind it.
The crackdown has led to international condemnation and sanctions, including from the U.S.
"What's happening in Syria is very uncertain and troubling because many of us had hoped that President Assad would make the reforms that were necessary," U.S. Secretary of State Hillary Clinton told reporters in Turkey on Saturday. "The brutality has to stop, there must be a legitimate sincere effort with the opposition to try to make changes."
Saturday's conference is part of a series of meetings the opposition has held recently in Turkey and Europe in an attempt to organize their ranks. Activists in Damascus also took part in Saturday's meeting by telephone.
Organizers had planned to hold a conference in Damascus in tandem with the Turkey meeting, but it was canceled after Friday's bloodshed. At least 14 of those killed Friday died near a hall in the Damascus neighborhood of Qaboun, where Saturday's conference was to be held.
Opposition figure Mashaal Tammo, addressing the conference by phone from Damascus, said Assad had lost his legitimacy to rule and called on him to step down.
In an emotional speech, he said the "the existence of the regime was no longer justified," and called for a peaceful transition to a civil, pluralistic and democratic state.
Al-Maleh at the conference accused Assad of leading a "fascist regime" and praised the "heroic people of Syria" rising up him.
Some participants at the conference had proposed the idea of forming a "shadow government" to immediately counter Assad's rule. But the idea was dropped after dissidents inside Syria said they feared being sidelined by the exiled figures.