BEIRUT, Lebanon - Cries of "Sinioria, out!" echoed across Beirut as hundreds of thousands of opposition protesters packed into the capital for a Hezbollah-led rally the army described as "unprecedented."

"Change is coming," the opposition banners boasted as a sea of demonstrators waving Lebanese flags spilled into the streets surrounding the offices of Western-backed Prime Minister Fuad Siniora.

Fully veiled Shiite women, Christian students wearing T-shirts and fathers hoisting children on their shoulders were among the crowds who cheered a series of opposition speakers urging the resignation of the anti-Syrian government.

Hezbollah chief Hassan Nasrallah, whose Damascus-backed movement has spearheaded the protests which kicked off December 1, has vowed his supporters will not quit the streets until the cabinet makes way for a government of national unity.

As many as 20,000 troops deployed on the streets and around the government building where Siniora has been holed up.

Hezbollah's Al-Manar television said the protest promised to be larger than the December 1 rally that kicked off the opposition campaign.

"This is a sea of demonstrators unprecedented in the history of Lebanon," an army spokesman said, estimating that "hundreds of thousands" had gathered in the heart of Beirut and on access roads to the city center.

In addition to red-and-white Lebanese flags, some demonstrators waved orange banners and photos of Christian former general Michel Aoun, as well as yellow Hezbollah flags and the green standards of the Shiite movement Amal.

As the deadlock deepened in a political crisis which many fear could plunge Lebanon back into civil strife, the opposition vowed to escalate its action and paralyze main roads, including the Beirut airport road.

Siniora warned against such actions, saying they "will put the country into a cycle of violence which will not be in anyone's interest."

The premier acknowledged that "our political and democratic regime is facing a challenge," but said that "Lebanon is a strong country -- we will overcome this crisis."

He again called for talks instead of protests. "Our hand is extended. We will not close any doors. We will work on opening doors."

Government supporters have staged large counter-rallies amid the deadlock that thtreatens to paralyze the legislative process with the spekaership of parliament in the hands of the opposition.

Arab envoys have also shuttled between Lebanese leaders in a bid to broker a settlement.

A US official warned on Saturday that the situation could deteriorate.

"If any one group, particularly with support from the outside and with the threat of violence, attempts to force its will on others, we think the situation will get significantly worse," James Jeffrey, principal deputy assistant secretary of state, told reporters in Kuwait.

King Abdullah of Saudi Arabia, a key financial backer of Lebanon, opened a Gulf summit Saturday with a warning that the Arab world was a powder keg waiting to explode.

"In Lebanon, we see dark clouds threatening the unity of the homeland, which risks sliding again into... conflict among the sons of the same country," he said.

The Lebanese opposition accuses the government of weakness and corruption, and says it no longer represents the people after six pro-Damascus ministers submitted their resignations last month.

The protesters want to replace the current cabinet, formed after 2005 elections, with a national unity government that they say is required by the power-sharing arrangements in force since the 1975-90 civil war.

Siniora, who has received messages of support from Britain, France, Germany, the United States, and some pro-Western Arab states, has pledged that what he branded an attempted "coup" by Hezbollah will fail.

The premier also expressed concern about the deployment to the capital of troops he said ought to be safeguarding Lebanon's borders after the July-August war between Hezbollah and Israel.

Siniora's coalition has accused the opposition of seeking to block a cabinet endorsement of plans for an international tribunal to try suspects in the 2005 murder of former premier Rafiq Hariri, widely blamed on Syria.

Pro-Syrian President Emile Lahoud on Saturday formally rejected the cabinet decision on the court, saying it should be "be reviewed by a legal, constitutional and consensual government." -AFP