The opposition groups were invited by Turkey and Qatar, which holds the rotating chair of the Arab League, to the talks to try to form a common front in the one-year uprising against Assad.
More than 300 dissidents listened to an opening address by Turkish Foreign Ministry official Halit Celik at the seaside hotel in Pendik, a suburb on the Asian side of the city.
"Turkey will not leave the Syrian people to their fate," Celik said. He said there was no alternative except for Assad's regime to go, and extended support to the Syrian National Council (SNC) umbrella group, as a platform for different strands of the opposition.
Opening proceedings were interrupted by Haitham al Maleh, a liberal Islamist and grand old man of the opposition, walking out of the hall after SNC president Burhan Ghalioun set out an action plan that called for greater unity.
Maleh, a former judge now in his 80s who has been jailed by both Assad and his father, said he was quitting the meeting because the SNC had assumed too much dominance and failed to let other activists have their say.
His walkout heralded expected fierce debates over the strategy to overthrow Assad, as well as on calls for reform of the SNC, delegates said.
The SNC's action plan, which included raising international backing, and support for peaceful protests, went further by proposing that it should help organise and arm the rebel Syrian Free Army, established by army defectors, to resist Assad's security forces, and raise money to pay recruits.
Ghalioun also called for backing for the one-day meeting to end with a "national oath", committing all the opposition to building a democratic state, without any agenda for revenge, and to seek reconciliation once Assad is removed.
A draft declaration said the new Syria will be "civic, democratic and totally free", with a transitional government to organise a ballot to elect a founding assembly to draft a new constitution.
"The Syrian people are proud of their cultural and religious diversity. Everyone will contribute in building the future," it said.
Some delegates complained that though the SNC has more than 300 members, only a handful take decisions and that while all sectarian and ethnic groups are represented on the executive, that was little more than tokenism.
"The executive council will have to do something to show it is listening to people," said a diplomat observing the meeting. "There is a feeling it is not transparent or democratic enough."
Some dissidents withdraw
A few leading dissidents, including Maleh, withdrew from the SNC in recent weeks, dismayed by its leadership and the influence of the Muslim Brotherhood, which draws support from Syria's Sunni majority.
This disunity has fed fears that Syria's agony will not end if Assad is pushed out, and the country could slide into sectarian and ethnic conflict, giving pause to governments which would otherwise be glad to see Assad's downfall.
Turkey hosts a "Friends of Syria" meeting of mostly Western and Arab foreign ministers on April 1 to try to agree measures to persuade Assad to call off his security forces, permit inflows of humanitarian aid and allow a political transition.
Whether they are in the SNC or not, main opposition figures will also attend, a Turkish official told reporters on Monday.
Opposition disunity is unsurprising, given that political life in Syria has been choked by 42 years of Assad family rule.
"This is a learning process in the politics of opposition," the Turkish official said.
Ghalioun, a Paris-based secular professor of politics, was chosen in October as a consensus candidate to hold the presidency for an initial three months, and has held onto the position despite strong criticism of his leadership.
His attempt in December to draft an accord between the SNC, made up mainly of exiled dissidents, and the National Coordination Body, a centrist bloc inside Syria, was rejected by the SNC executive council.
Ethnic Kurdish delegates also complained over the way the SNC was run. They said the notion of a civic state was too woolly, and favour commitment to a secular state that recognises women's rights and the place of Kurds in Syrian society.