Hundreds of thousands of Lebanese heeded Hezbollah's call, flooding downtown Beirut to pressure the US-supported government to resign.

But Prime Minister Fuad Saniora showed no signs of backing down in a confrontation that could tear apart the country.

The protesters raised a deafening noise of chants and revolutionary and nationalist songs just outside the main government offices on Friday.

Barbed wire and armoured vehicles separated the demonstrators from the buildings where Saniora and some of his ministers were holed up.

"Pack your bags Fuad, and leave us and the country in peace," many shouted.

The loud but peaceful rally, which took on a carnival-like atmosphere in bright autumn sunshine, was far larger than last week's pro-government demonstration that followed the assassination of Industry Minister Pierre Gemayel.

Friday's crowd was estimated at between 800,000 and a million people, who formed a sea of red-and-white Lebanese flags with the green cedar emblem.

The protest was the opening volley in the Shiite Muslim group Hezbollah's campaign of open-ended demonstrations.

The event could be a watershed for the future of Lebanese politics, torn between anti-Syrian politicians who control the government and pro-Syrian forces led by Hezbollah.

Hezbollah and its allies demand a third of the seats in Saniora's Cabinet - enough to veto its decisions - and have vowed to continue their campaign until the government falls.

They cite Lebanon's constitution, which says the cabinet is supposed to represent all communities in the country. (AP)