Hundreds of thousands of supporters of Hezbollah and its pro-Syrian allies have held a mass rally in Beirut to protest against Lebanon's government.
As night fell, several thousand set up tents outside PM Fouad Siniora's office for an open-ended sit-in.
The opposition says it will keep up the pressure until the government resigns.
The protest follows weeks of rising tension in Lebanon, with the killing of a leading anti-Syrian politician and resignations from the cabinet.
The huge crowd turned central Beirut into a sea of red and white Lebanese flags.
Hezbollah leaders had asked people not to wave the yellow flag of their movement, which features a fist holding a Kalashnikov rifle.
The noisy but peaceful crowd filled Riad Solh Square, in front of Mr Siniora's office, says the BBC's Simon Wilson in Beirut.
Mr Siniora and some of his ministers were inside, just metres away, and will have heard the deafening music and speeches, our correspondent says.
The army and police mounted a large security operation, closing off the prime minister's office and other key buildings with barbed wire and armoured vehicles.
During the afternoon, the huge crowd listened to Hezbollah songs and speeches.
Police estimated its size at 800,000 people, but Hezbollah claimed it was larger, the Associated Press news agency said.
Lebanese troops were placed at key government buildings
Speaking from behind a bullet-proof glass screen, Christian opposition leader Michel Aoun told the cheering crowd that the current government was unconstitutional and should resign.
He said they had "made corruption a daily affair".
Mr Siniora's government has vowed to stand firm against what he has called an attempted coup.
"Lebanon's independence is threatened and its democratic system is in danger," he said on Thursday.
Meanwhile, US State Department spokesman Tom Casey expressed concern, saying "Hezbollah and its allies, with support from Syria and the Iranian government, are continuing to work to destabilize Lebanon".
US Ambassador to the UN John Bolton called the Hezbollah demonstration "part of the Iran-Syria inspired coup".
Under Lebanon's constitution, the death or resignation of another two ministers will automatically topple it.
Hezbollah has been demanding a bigger share in the cabinet that would give it the power to veto government decisions.
The government came to office last year in the first election held after the withdrawal of Syrian troops originally stationed in Lebanon during the civil war.
Mr Siniora warned any attempt to topple his government would fail
Syria was forced to withdraw its military presence after massive street protests and international pressure, triggered by the February 2005 assassination of former Lebanese Prime Minister Rafiq Hariri.
A UN investigation has implicated several Syrian officials in the killing, although Syria has denied any involvement.
The government in Beirut has also accused Damascus of ordering the assassination on 21 November of anti-Syrian cabinet minister Pierre Gemayel.