Fouad Siniora, the Prime Minister, says that the Government is determined to remain in power, accusing the Hezbollah-led opposition of attempting to mount a coup and acting on the orders of Syria and Iran.

With neither side willing to give way, many Lebanese fear that the political deadlock will be broken only by violence.

The huge crowd, numbering perhaps 800,000 or almost a quarter of the population, packed two squares in the city centre. They had travelled from all over Lebanon.

“Siniora out, Siniora out,” chanted groups of young supporters of General Michel Aoun, the former Lebanese army commander who is an opposition ally of Hezbollah.

General Aoun is also a Christian leader, emphasising that opposition to the Sunni-led Government goes wider than Shia Hezbollah.

Highlighting the sectarian divisions in Lebanon’s power struggle, King Abdullah of Saudi Arabia, a predominantly Sunni country, telephoned Mr Siniora and every member of his Cabinet to offer his personal support.

Mr Siniora and his ministers sat out the rally in the heavily protected Grand Serail, his offices overlooking the city centre. Troops had slung rows of razor wire across the approach roads, increasing the impression of a government under siege.

Mr Siniora has increased hugely the numbers of security personnel around him amid assassination fears. Last month Pierre Gemayel, the Industry Minister, died in a hail of bullets aimed at his car.

“I call on the Prime Minister and his ministers to quit,” General Aoun said, to the cheers of protesters. “I wish that the Prime Minister and his ministers were among us today, not hiding behind barbed wire and army armoured carriers. He who has his people behind him does not need barbed wire.”

About 20,000 troops were deployed on the streets to forestall any outbreak of trouble. Armoured personnel carriers equipped with heavy machineguns were parked at road junctions. About 15,000 Hezbollah marshals channelled Shia Muslim protesters along the main roads from the southern suburbs of Beirut to the city centre, ensuring that none strayed into Christian neighbourhoods.

“We want to topple the American government in Beirut,” said Iman Shehadi, wearing a full-length black chador, who had travelled from Bint Jbeil in south Lebanon. “We want a government of people willing to sacrifice themselves for Lebanon,” she added.

The turmoil has been festering since the end of the war between Hezbollah and Israel four months ago. The catalyst was the imminent formation of a tribunal under UN auspices to try those accused of murdering Rafik Hariri, a former Lebanese prime minister who died in a truck bomb explosion in February last year.

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A UN commission has suggested that Syria was responsible and the anti-Syrian Lebanese Government believes that Damascus has instructed its allies in Lebanon to create a political crisis to forestall the establishment of the tribunal.

But bigger issues are at stake. Lebanon has become an arena for a struggle for control of the Middle East, pitting the West and its Arab allies, such as Saudi Arabia and Egypt, against an anti-Western alliance grouping Iran, Syria and Hezbollah.

The US views the Lebanese Government as a useful tool to maintain pressure on neighbouring Syria and to block Iran’s ambitions to influence directly the Arab-Israeli conflict through Hezbollah, its ally.

But the anti-Western axis is seeking to weaken US influence in Lebanon by replacing Mr Siniora’s Government with one more sympathetic to Damascus and Tehran. Inevitably, the political struggle in Lebanon takes on a sectarian edge, with the Government supported by most Christians and the Sunni and Druze communities against an opposition overwhelmingly composed of Shia Muslims. Walid Jumblatt, the leader of the Lebanese Druze and a harsh critic of Syria, said that the rally was an “attempted coup, but we will remain strong”.

Hezbollah says that it will increase its actions if the Government refuses to yield to the opposition’s demands, raising fears of a prolonged crisis.

“It will take some weeks,” said Michel Samaha, a former Lebanese minister and Syrian ally who attended the rally.

“I hope that change comes about through compromise, if not we are going to have real problems.”

What's next for Lebanon?

If the opposition triumphs

If the Government survives

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