Hundreds of thousands of Hizbullah-led protesters swarmed downtown Beirut on Sunday, demanding Prime Minister Fuad Saniora cede some power to the opposition or step down. The demonstration could be a tipping point in Lebanon's burgeoning political crisis, ten days after a coalition of largely pro-Syrian opposition groups launched a series of rallies against Saniora's anti-Syrian, US-backed government. Holed up in his fortified office downtown, the prime minister spoke Sunday by video link to a memorial for an anti-Syrian politician killed in a car bomb last year, and addressed the political climate and protests. "What is the great cause for this tense political clamoring and the open sit-ins?" Saniora said. "Is this the ideal way to achieve demands, whatever they are?" He said he was open to dialogue between his government and the opposition, and acknowledged that the political crisis threatened Lebanon's security, economy and political system. "We don't want Lebanon to be an arena of the wars of others. Lebanon is a nation, not an arena," Saniora said, in a veiled reference to Hizbullah's backers in Syria and Iran. Lebanese combat troops and armed police sealed off major roads and added more layers of barbed wire around the prime minister's sprawling downtown complex, where he has been living along with most of his ministers since Dec. 1. The political unrest has split the country along dangerous sectarian lines, with most Sunni Muslims supporting the Sunni prime minister and Shiite Muslims backing the militant group Hizbullah. Christian factions are split between the two camps. Thousands of demonstrators camped out in two downtown Beirut squares overnight, and hundreds of thousands more joined the crowd for the afternoon demonstration. Several hundred tents have lined the area for more than a week. Police had no immediate crowd estimates, but the rally - filling downtown Beirut's plazas and many neighborhoods - promised to be one of the biggest in Lebanon's history. Protesters streamed downtown, waving Lebanese and Hizbullah flags as loudspeakers blasted anti-government speeches and anthems in support of the guerrilla group. Bands of musicians pounded drums in a carnival-like atmosphere, while Hizbullah security agents wearing white caps fanned out in the crowd. "Down with the corrupt government," read one banner. "We want a clean government," read another. "Let the government fall!" an organizer shouted through a loudspeaker, with the crowd roaring after him in approval. Addressing Saniora, he said: "Do you want blood or what?" "We have come to show them how big our size really is," said Reem al-Zein, a 20-year-old philosophy student wearing a Muslim headscarf. "I think this lying government will not be able to last much longer after today." Lebanon's political crisis began after talks over a national unity Cabinet collapsed, and Hizbullah's two ministers and four allies resigned from the Cabinet and joined the opposition. It erupted Nov. 21 with the assassination of anti-Syrian politician Pierre Gemayel, followed by a national strike, his funeral and the opposition sit-in. Street protests have since paralyzed the core of Beirut. A Shiite Muslim supporter of the opposition was shot dead in a Sunni Muslim neighborhood on his way home from protests a week ago. Saniora has refused to quit and has received hundreds of supporters daily at his office complex to counter the opposition protests and sit-ins outside. He and Hizbullah leader Sheik Hassan Nasrallah have exchanged unprecedented accusations and insults. Tension had been brewing for months, and relations between the two camps deteriorated after the Israel-Hizbullah war last summer and a UN draft for the creation of an international tribunal to try suspects in the 2005 assassination of former Premier Rafik Hariri. The summer war ravaged parts of Lebanon. Hizbullah's fight against Israel sent its support among Shiites skyrocketing, emboldening it to grab more political power. Hizbullah now accuses Saniora and some elements in his government of working with Israel to destroy the guerrilla force. Pro-government groups, in turn, resent Hizbullah for sparking the war by snatching two Israeli soldiers. They, along with the United States, accuse Hizbullah's Syrian and Iranian backers of seeking to overthrow the government. On Sunday, the crowd of protesters gathered under a giant banner depicting Saniora kissing U.S. Secretary of State Condoleezza Rice on the cheek on a visit to Beirut during the war. Written in English on it was: "Thanks Condy." Hizbullah and its allies fault the US for refusing to push the UN Security Council to call an immediate cease-fire. American officials said they wanted to make sure a truce would stick before pushing for an end to the fighting, which lasted 34 days and killed more than 850 people on both sides. President Emile Lahoud on Saturday refused to endorse the draft accord sent to him by Saniora's divided Cabinet to create the international tribunal. He maintained that the remaining Saniora Cabinet had lost its constitutional legitimacy, an argument the prime minister has disputed because Cabinet meetings still have the quorum necessary to make decisions. The president's action was certain to intensify tension. Mass protests also followed the 2005 slaying of Hariri, which forced Syria to end a nearly three-decade military occupation of Lebanon. A UN investigation has said the attack's complexity suggested the Syrian and Lebanese intelligence services played a role in the assassination. Syria denies involvement.