BEIRUT, Lebanon -- Hundreds of thousands of Hezbollah members and their allies flooded central Beirut on Sunday demanding changes in the government's makeup as soldiers strung more barbed wire around the offices of the Western-backed premier.
Buoyed by the big turnout after a week of street protests, the pro-Syria opposition gave Prime Minister Fuad Saniora an ultimatum of a "few days" to accept its demand to form a national unity government with a big role for Hezbollah or face an escalating campaign to oust him.
Saniora, who has been holed up in his fortified office downtown, rejected the demand and urged his foes to resume negotiations. The opposition should "return to the constitutional institutions to discuss differences and reach real solutions," he said in a written statement.
Political unrest has split the country along sectarian lines, with most Sunni Muslims supporting the Sunni prime minister and Shiite Muslims backing the militant Hezbollah. Christian factions are split between the two camps.
But despite the heated rhetoric of the political confrontation, Sunday's mass gathering remained peaceful and left the door open to the possibility of a settlement.
"Hopefully it won't be long. At the end, there will be no winner, no vanquished. We should all be winners," Saad Hariri, leader of parliament's anti-Syria majority and a Saniora supporter, said.
Police had no immediate crowd estimate, but the horde that jammed downtown plazas and many neighborhoods appeared one of the biggest in a country that has seen a string of mammoth demonstrations by both sides in recent years. A Hezbollah anti-government rally Dec. 1 drew 800,000 of Lebanon's 4 million people, according to police figures.
Pro-government groups staged a rival demonstration that drew tens of thousands in the northern port city of Tripoli.
Hezbollah's supporters streamed into downtown from all corners of Lebanon, waving Lebanese and Hezbollah flags as loudspeakers blasted anti-government speeches. Musicians pounded drums in a carnival-like atmosphere, while Hezbollah security agents fanned out in the crowd.
"We have come to show them how big our size really is," said Reem al-Zein, a 20-year-old philosophy student. "I think this lying government will not be able to last much longer after today."
Sheik Naim Kassem, Hezbollah's deputy leader, said the opposition was willing to stay on the streets for months to achieve its goal.
"Does Bush want popular expression in Lebanon? Do the West and the Arabs want to hear the voice of the people in Lebanon? Tell them 'Death to America!' Tell them 'Death to Israel!"' the crowd repeated behind him.
Michel Aoun, a Christian leader allied with Hezbollah, warned Saniora he had only "a few days" to accept a national unity government or face further action.
"What we hope for today is for them to understand that their era is over," he said in a video link shown on giant screens.
Aoun added that the opposition was committed to "to peaceful means, but even other means are legitimate."
Lebanese media have speculated the opposition's next steps could include civil disobedience, disruptions of public services and resignations from parliament.
The political crisis began after talks on a national unity government collapsed and six pro-Hezbollah ministers resigned from the Cabinet. Hezbollah is demanding that it and its allies hold a third of the Cabinet posts, which would allow them to veto government decisions.
Relations between the two camps deteriorated after the Israel-Hezbollah war last summer and a U.N. push for an international tribunal to try suspects in the 2005 assassination of former Lebanese Premier Rafik Hariri, which many people blame on neighboring Syria.
Hezbollah's fight against Israel sent its support among Shiites skyrocketing, emboldening it to push for more political power. The militant group also accuses Saniora and some elements in his government of working with Israel to destroy the guerrilla force.
Pro-government groups, in turn, resent Hezbollah for sparking the war by capturing two Israeli soldiers. Saniora's allies, along with the United States and others, accuse Hezbollah's Syrian and Iranian backers of seeking to overthrow the government.
If Saniora resigned, he would continue in a caretaker capacity until parliament named a new premier and formed a new Cabinet, but he has shown no sign of giving in. Soldiers and police sealed off major roads in Beirut and strengthened defenses around Saniora's downtown complex, where he has been living with other Cabinet ministers.
On Sunday, Saniora said he was open to dialogue and acknowledged 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," Saniora said, in a veiled reference to Syria and Iran. "Lebanon is a nation, not an arena."
Saad Hariri, son of the assassinated premier, said the government was open to meeting opposition demands but only as part of a broader deal.
Among those, he said, would a decision on the fate of President Emile Lahoud. Anti-Syria groups have repeatedly demanded his resignation, accusing him of being a front for Syria, whose army occupied Lebanon for nearly three decades.
Hariri said the opposition also would have to agree to support the international tribunal on his father's slaying and to implement all of the U.N. resolution that ended the summer war. One of the latter's requirements is for Hezbollah to disarm, something it has refused to do.
"Once we agree on the principles there is no problem," Hariri told AP.