Nearly one million people gathered for an opposition rally in Beirut, officials say - a month after the death of former Prime Minister Rafik Hariri.
The demonstration surpassed recent pro-Syrian rallies and is thought to be the biggest in Lebanese history.
The BBC's Kim Ghattas in Beirut says the crowds turned the city centre into a sea of red, white and green - the colours of Lebanon's national flag.
They were protesting against the presence of Syrian forces in Lebanon.
Syria has pulled back some of its troops in Lebanon to the border.
Damascus also promised the UN a full timetable for the withdrawal of its 14,000 troops and intelligence agents.
Meanwhile military intelligence officers left two offices in the north, in the town of Amyoun in Koura region and Deir Ammar on the coast, the Associated Press news agency reported.
Beirut city official Mounib Nassereddine told AFP news agency more than 800,000 people had turned out for the protest, which would make it the biggest demonstration held in the country's history.
AP also estimated the same turnout.
Demonstrators packed Martyrs Square, near where Hariri died in a car bomb, and the crowd spilled over into nearby streets.
The sea of people fell silent at 1255 (1055 GMT), the exact time Hariri was killed four weeks ago.
Unlike previous anti-Syrian rallies, Sunni Muslims came out in force to join Druze and Christians to commemorate the loss of their leader.
A stream of buses and cars brought protesters from the eastern Bekaa Valley, while others arrived from Junieh in the north by sea.
"Hezbollah organised a giant demonstration last Tuesday to intimidate us," Nada, 35, who travelled to Beirut from Zahle in the east, told AFP.
"Today we're taking up the challenge and invite [Hezbollah] to join us because we represent the true majority of the country," she said.
Many offices and schools closed early for the demonstration.
Our correspondent says a lot more effort went into organising this event, and the media owned by Hariri was a driving force.
The opposition will try to keep the momentum until parliamentary elections in May, she adds.
An investigation is continuing into Hariri's death and the opposition has demanded the resignation of senior security officials.
Druze opposition MP Marwan Hamadeh told the huge crowd: "You want the truth [about Hariri's killing]? It's clear... the world and Lebanon know them [the killers] well, know them one by one, name by name, rank by rank."
Pro-Syrian rallies have been held in recent weeks
But Hariri's sister drew jeers from the crowd when she told them: "We will stand by Syria until its land is liberated and it regains its sovereignty on the [Israeli-] occupied Golan Heights."
Correspondents say fears are growing that protests could spill over into violence, with the authorities considering a ban on protests after Monday.
Both President Emile Lahoud and Maronite Christian Patriarch Nasrallah Sfeir, a leading opposition mentor, have urged dialogue as an alternative to protests as a way out of the crisis.