Hundreds of thousands of protestors, led by the pro-Syrian militant group Hezbollah and its allies, have gathered on the streets of Beirut demanding the resignation of the Western-backed Lebanese government.

The crowd of almost one million was met by thousands of combat troops, who have been deployed across the capital amid fears today’s events could spiral into violent clashes.

The unprecedented security measures also saw 16 armoured carriers patrolling Beirut, while armed police earlier sealed off the offices of Prime Minister Fouad Siniora with barbed wire and barricades.

Holed up in his headquarters, the Prime Minister went about his schedule in an apparent attempt to ignore the masses who arrived from all corners of the country to join the demonstration, which the government has described as an attempted coup to which it will not yield.

As some of the hordes surrounded Mr Siniora's office, others congregated on the streets, waving Lebanese flags as organisers had instructed. The protest is due to be followed by an indefinite sit-in, with opposition leaders saying they would encircle the government’s headquarters to stop movements to and from it until the Prime Minister caved in.

"I call on the prime minister and his ministers to quit," Michel Aoun, Christian opposition leader, said to an outbreak of cheers.

"Out, out Siniora," was one of the many cries. "We have had enough ordeals and tears. Siniora, out – we want a free government. We want a government that will feed our hunger."

Sheikh Naim Kassem, Hezbollah deputy chief, said that the campaign would continue until the cabinet fell. "This government will not take Lebanon to the abyss. We have several steps if this government does not respond but I tell them you will not be able to rule Lebanon with an American administration," he told Hezbollah’s al-Manar television.

Last night, Mr Siniora vowed that his government would not be brought down by the opposition. Addressing the nation in a televised speech, a defiant Prime Minister warned that "Lebanon’s independence is threatened and its democratic system is in danger," in a reference to Syria’s 29-year military presence, which only came to an end in 2005 after mass demonstrations in the wake of the assassination of former Prime Minister Rafik Hariri.

"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."

His statement was echoed today by Walid Jumblatt, Druze leader and prominent anti-Syrian MP, who appealed to his supporters to stay calm. "This is an attempted coup but we will remain strong," he said. "We will stay home…and when they will decide to return to dialogue, we will welcome that."

As well as Hezbollah, the opposition factions include the fellow-Shia Amal party, the Christian faction of Mr Aoun and supporters of the Syrian-backed president, Emile Lahoud.

Local media have labelled the protest as the "great showdown" between the government and the pro-Syrian opposition. Beirut has already been the centre of a number of previous demonstrations, from both the pro-Syrian and anti-Syrian lobby, the later of which has been pushing for more seats in the Cabinet, thereby giving it the power to veto decisions.

The two factions have reached a dangerous deadlock which threatens to paralyse all state institutions and has deeply divided the country.

Hezbollah has accused the government of pandering to the West, criticising Mr Siniora’s for failing to act during its war with Israel over the summer.

That conflict dramatically increased support for the militant group, which has shown in previous rallies that it is capable of drawing hundreds of thousands of its supporters into the streets.

The anti-Syrian lobby meanwhile, has charged the opposition of trying to bring down the government in order to derail a UN tribunal to bring to justice those responsible for Mr Hariri’s assassination, which has been widely blamed on Syria.

President Lahoud, a supporter of Damascus, has refused to recognise Mr Siniora’s government after the resignations of pro-Syrian six ministers, accusing it of being unconstitutional.

Troops deployed on the streets have been instructed not to takes sides in today’s protest, which organisers have insisted will be peaceful. Hezbollah has itself deployed its own so-called "discipline men" to maintain crowd control among between army forces and protesters.

Opposition groups had threatened to stage demonstrations last week, but postponed their plans after the assassination of anti-Syrian minister Pierre Gemayel on November 21. His death made Mr Gemayel the sixth critic of Damascus to be killed in the past two years.

His funeral saw hundreds of thousands of government supporters demonstrate two days later in an outpouring of anger at Syria and its allies in Lebanon, including Hezbollah.

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