The protest on Tuesday dwarfed previous protests demanding that Syrian troops quit Lebanon.
Hizb Allah chief Shaikh Hasan Nasr Allah urged the Lebanese opposition to join a national unity government and reject a UN demand for the Syrians to leave and his own militia to disarm.
Nasr Allah said no one in Lebanon feared the United States, whose troops left Beirut in 1984, a few months after a bomber killed 241 marines at their headquarters in the capital.
"We have defeated them in the past and if they come again we will defeat them again," he said, drawing chants of "Death to America" from the sea of demonstrators.
As the mainly Shia Muslim crowds thronged Riad al-Sulh Square, Syrian forces began moving eastwards under a phased withdrawal plan announced on Monday, the Lebanese army said.
The huge Hizb Allah rally was the first major show of popular support for Syria in Beirut since the 14 February assassination of former Lebanese prime minister Rafiq al-Hariri touched off daily anti-Syrian protests, mainly involving Maronite Christians.
Those protests, which drew tens of thousands on Monday, have been taking place in Martyrs Square, just 300m from the scene of the gathering organised by Hizb Allah and its allies.
The rival rallies, each using the Lebanese cedar flag to show patriotism, reveal deep rifts in Lebanon over Syria's role and the future of Hizb Allah, the country's last armed militia.
Syrian soldiers are due to pull
back towards the border
Hizb Allah officials and a pro-Syrian security source said one million people attended the rally and witnesses said the crowds were certainly in the hundreds of thousands.
Nasr Allah said he had no problem with a Syrian pullout under the 1989 Taif Accord that ended Lebanon's 1975-90 civil war, but would have no truck with a UN resolution adopted in September that called for a Syrian withdrawal and militia disarmament.
"If the mechanism for Syria's stay or withdrawal is within the Taif Accord then we are agreed," he told the rally.
"But those who insist on [Resolution] 1559, we say to them your insistence is a revolt against the Taif Accord ... and that means a revolt against national consensus," he declared.
Nasr Allah also urged France, which co-sponsored 1559 with the United States, to drop its support for the measure.
Shia Muslims, Lebanon's largest community, condemned al-Hariri's killing but few have joined Christian, Druze and Sunni Muslim opponents of Syria's dominant role in the country.
The Shia and many other Lebanese are proud of Hizb Allah, which is backed by Syria and Iran, for forcing Israel to end its 22-year occupation of south Lebanon in 2000.
"If the mechanism for Syria's stay or withdrawal is within the Taif Accord then we are agreed. But those who insist on [Resolution] 1559, we say to them your insistence is a revolt against the Taif Accord ... and that means a revolt against national consensus"
Hasan Nasr Allah,
Hizb Allah leader
Popular agitation in Lebanon, combined with intense world pressure, has prompted Syria to announce plans to end its 29-year military presence in its smaller neighbour.
"Implemented immediately from this date is the ... withdrawal of Syrian forces from the existing areas to the Bekaa area," a Lebanese army statement said.
Witnesses reported troops on the move in several places on
mountain ridges east of Beirut.
Syrian President Bashar al-Asad and his Lebanese counterpart Emile Lahud agreed on Monday to shift Syrian troops to eastern Lebanon by 31 March.
A statement said the Syrian and Lebanese military would then decide how long the Syrians would stay.
A Syrian official source in Damascus said Syrian security and intelligence agents would leave along with the troops.
Lebanese and foreign critics say Syrian intelligence controls Lebanon behind a facade of state institutions.
Hizb Allah, declared a terrorist group by Washington, began as an anti-Israel militia but is now also a political party with deputies in parliament and a network of charities.
Anti-Syrian demonstrations have
been going on for weeks
Opposition leader Walid Jumblatt called for dialogue with Hizb Allah, but said Syria must declare a withdrawal deadline.
"We want a clear-cut timetable for the pullout of Syrian troops," the Druze leader said after meeting German Foreign Minister Joschka Fischer in Berlin.
Syrian forces are credited with helping end the civil war that tore Lebanon apart.
Christian, Muslim and Druze militias fought each other and about 150,000 people are thought to have died in that conflict.