A former Syrian vice-president and the head of the Muslim Brotherhood attended the gathering on Thursday
Abdel-Halim Khaddam, the former vice-president who broke with Assad, the Syrian president, last year after serving under his late father Hafez al-Assad, was involved in the talks with nationalists, liberals, Islamists, Kurds and communists, participants said.
Husam al-Dairi, Washington-based leader of the Syrian Liberal National Democratic Party said "This is the first time in history that all the opposition movements inside and outside Syria have sat down at one table and agreed on a common plan".
He said the coalition of 25 opposition individuals and movements, including the London-based Muslim Brotherhood secretary-general, Ali Saad-al-din Bayanouni, would elect a leader at the Brussels meeting and announce its programme on Friday.
Khaddam, who lives in France, chose to stage the meeting in Belgium because he is bound by French law to refrain from political activities under the terms of his political asylum, his son Jihad Khaddam said.
"The Syrian people can no longer stand the pressure of the regime and is going to revolt," Jihad Khaddam said, adding that his father had vowed to return home to Damascus after a revolution before the end of this summer.
"The coalition is open to everyone. Of course we cannot name the supporters inside for their own safety," he said.
He said the exiled opposition counted many supporters within the ruling Baath Party and believed the army would remain neutral if a popular protest movement arose against what he called "the thin layer of dictatorship".
Pressure on Assad and his family has mounted since a UN investigation into the assassination last year of Rafik al-Hariri, a former Lebanese prime minister, implicated top Syrian officers and asked to question senior officials.
But despite mass demonstrations in neighbouring Lebanon which led to the withdrawal of Syrian troops after Hariri's murder, there has been little sign of popular protest inside tightly controlled Syria.
Assad and his vice-president have agreed for the first time to talk to the UN inquiry, the world body said this week.
The United States and France have been in the lead in putting international pressure on Syria over the Hariri case.
But some diplomats say Washington and Paris may be reluctant to risk instability in Damascus at a time of worsening civil strife in neighbouring Iraq and after the victory of the Hamas movement in the Palestinian territories.