Beirut, Lebanon ? They came on foot and by bus. They jammed streets and climbed onto rooftops, mosque minarets and fences, thronging Beirut to roar anti-Syrian chants. And they fell silent at 12:55 p.m., the exact time former Prime Minister Rafik Hariri was assassinated a month ago.

Drawn by e-mails and cell phone text messages, hundreds of thousands of people from across Lebanon marched in the streets of the capital, waving a sea of white and red Lebanese flags and shouting “Syria out.”

The rally, perhaps the biggest anti-government demonstration ever staged in the Arab world and certainly the biggest ever in Lebanon, was the opposition’s bid to regain momentum. It came after two serious blows: the reinstatement of the pro-Syrian prime minister and a huge rally last week by the Shiite Muslim group Hezbollah to voice support for Syria.

Protesters — some bused in from across Lebanon — jammed Martyrs’ Square and spilled into nearby streets. They chanted, sang and shouted in a mix of the Arabic accents of the country’s regions, demanding that Syrian troops depart and that their government be purged of Damascus’ influence.

The turnout was broader than earlier opposition protests, with more Sunni Muslims in particular joining the Christians and Druse who have formed the bulk of past anti-Syrian rallies. Even some Shiites joined in.

“We came to liberate our country. We are coming to demand the truth,” said Fatma Trad, 40, a Sunni woman wearing a headscarf. “I’ve been watching it all on television for the past month. Today, I wanted to be a part of it.”

As Syria pulls its troops toward the border for an eventual withdrawal from the country it has controlled for decades, both the pro-Syrian government and the opposition have been whipping up crowds in a duel of people-power one-upmanship.

Each side seeks to show it has the louder voice of the people behind it. For the opposition, Monday’s rally was vital to demonstrate it could claim the street after Hezbollah’s March 8 rally drew a half-million people. The Shiite group has organized large rallies in the past, but its showing last week was a sign of its determination to make sure no future Lebanese government would consider peace with Israel or pressure Hezbollah to disarm.

In recent days, opposition ads for Monday’s rally ran on television, and activists in towns and villages arranged buses to the capital.

E-mails and telephone text messages referred to Prime Minister Omar Karami’s claim that the Hezbollah demonstration showed the government had the support of the majority. “Prove him wrong,” the messages flashed.

Lebanese opposition protesters carry an anti-Syrian banner reading Syria