Lebanese Prime Minister Fouad Siniora has said his government will not be brought down by mass protests planned by pro-Syrian opposition groups.
Siniora warned any attempt to topple his government will fail
Speaking live on national television, Mr Siniora said: "We will not allow any coup against our democratic system."
His remarks came ahead of a huge anti-government demonstration called by the Hezbollah movement and its allies.
Thousands of Lebanese troops and police have been deployed across central Beirut to tighten security.
As well as a heavy troop presence in the centre of the Lebanese capital, ground forces, backed by tanks and armoured vehicles, have also taken up positions at the main entrances to the city.
Tensions in Lebanon soared after a leading anti-Syrian minister, Pierre Gemayel, was killed on 21 November and pro-Syrian MPs quit the cabinet.
Hezbollah has accused the Mr Siniora of leading a government which has failed in its mission.
"Fellow Lebanese, we have ahead of us decisive days for Lebanon and its future. In his address, Mr Siniora appealed to Lebanese to stand firm in the face of threats against his government.
"We will not allow any coup against our democratic regime. We are determined to stay the course, as our government is legitimate and constitutional... and enjoys the confidence of parliament," he said.
Lebanon's pro-Syrian President Emile Lahoud has declared the government unconstitutional following the resignations earlier this month of five Shia ministers and one Christian ally.
The ministers quit after Hezbollah's call for more cabinet seats for it and its allies was rejected.
Hezbollah has been threatening to hold mass protests for weeks in support of its demand, which would effectively give it a veto over decisions.
Earlier on Thursday, Hezbollah leader Sheikh Hassan Nasrallah also made a televised address, urging Lebanese to join protests.
"We appeal to all Lebanese, from every region and political movement, to take part in a peaceful and civilised demonstration on Friday to rid us of an incapable government that has failed in its mission," he said.
The pro-Syrian opposition has also condemned the cabinet's decision to approve a plan for an international tribunal to try suspects in the murder of former Prime Minister Rafik Hariri.
A UN inquiry has implicated Syrian officials in Mr Hariri's death in 2005, an allegation Syria strongly denies.