But Hezbollah leaders signaled they weren't looking for a bloody showdown by pulling back their fighters late in the day. The group, and gunmen from allied groups, also steered clear of government buildings and made no attempt to advance toward Beirut's Christian area.
The Western-backed government, which holds only a small majority in parliament, and opposition parties led by Hezbollah have been deadlocked for 17 months over the government's course.
Sporadic street clashes had broken out the past year. But combat erupted this week after the Cabinet sought to rein in Hezbollah by ordering the removal of an airport security chief with ties to the group and demanding the dismantling of the movement's private phone network.
The quick humiliation of Sunni fighters - who are far less organized than Hezbollah's militia - showed the Shiite group is more than strong enough to prevent actions it opposes.
At one point Friday, about 100 Hezbollah militants wearing matching camouflage uniforms and carrying assault rifles marched down the capital's main commercial street in a display of might meant to show Prime Minister Fuad Saniora's government who really is in charge.
The army largely stood aside as Shiite fighters scattered their opponents in street battles across the capital's Muslim sector. Lebanon's generals have stayed out of the conflict in fear that intervening could splinter the army along sectarian lines, as happened in the devastating 1975-90 civil war that killed 150,000 people and lefts parts of Beirut a moonscape.
At least 15 people were reported killed since Wednesday - the worst sectarian bloodshed since the civil war.
White House spokesman Gordon Johndroe blamed Hezbollah and its backers for the violence. "The Hezbollah terrorist organization, aided by its Iranian and Syrian sponsors, continues to undermine Lebanon's sovereignty and democratic institutions," he said.
Johndroe said the U.S. was consulting with other governments in the region and with the U.N. Security Council about measures to hold those responsible for the fighting accountable.
The rout of Sunnis was a blow for Washington, which has long considered Hezbollah a terrorist group and condemns its ties to Syria and Iran. The Bush administration has been a strong supporter of Saniora's government and its army the last three years.
Hezbollah's show of military power was certain to both strengthen its own political position and deeply worry a Middle East and Western world that are nervous about Iran's growing influence and its intentions in the region.
"The government tried to show force by shutting down Hezbollah. Hezbollah showed force by pushing back the government," said Jon Alterman, director of the Center for Strategic and International Studies Middle East Program in Washington.
"Hezbollah emerged stronger and the government emerged weaker," Alterman added.
The group's fighters took up positions on street corners and along sidewalks, flagging down the few cars that ventured out on nearly deserted roads to search their trunks.
Nearby, dozens of gunmen from an allied party appeared, some covering their faces with masks and carrying rocket-propelled grenade launchers. Elsewhere, Hezbollah-allied militiamen from the Syrian Social Nationalist Party drove in cars, firing in the air in celebration.
Hours after Hezbollah's takeover, the sounds of shooting decreased, and Hezbollah fighters began withdrawing to Shiite neighborhoods, speeding off in pickup trucks and SUVs as Lebanese troops replaced them. But some Shiite gunmen remained on street corners or sat inside parked vehicles.
Christian leader Michel Aoun, a close ally of Hezbollah, declared after Hezbollah's triumph that "the train is back on the right track" and predicted the situation would de-escalate.
The pullback indicated Hezbollah did not intend a lasting takeover of the Sunni Muslim parts of Beirut, unlike the seizure of the Gaza Strip a year ago by the Islamic militant group Hamas. Hezbollah's leaders likely are wary of pushing too far in a nation whose people are divided among 17 Christian and Muslim sects.
The political crisis has its roots in a split among Lebanese over Syrian and Iranian influence in their country. Hezbollah and its allies are friendly with those two nations, while the factions united behind Saniora look to the West and accuse Syria and Iran of using Lebanon to push their agenda for the Middle East.
Lebanon's top Shiite cleric, Grand Ayatollah Mohammed Hussein Fadlallah, said he was confident Lebanon will not drift into widespread Iraq-style sectarian bloodshed.
But the street violence stunned many in the country, leaving streets empty and people huddled inside their homes in fear and uncertainty.
"I'm shocked," said Iman Humaydan, a 51-year-old Druse author. "It reminds me of 1982 when Israel invaded Lebanon and caused hatred among the people. And now once again people feel hate."
Saniora and five Cabinet ministers were at the heavily protected prime minister's compound in downtown Beirut.
Other officials of the pro-government coalition met in the Christian heartland north of the city. Afterward, pro-government Christian leader Samir Geagea read a statement accusing Hezbollah of attempting a coup so Syria and Iran can control Lebanon.
"Violence will not terrorize us, but it will increase our resolve," he said.
Nationals from several countries, including Kuwait and the United Arab Emirates, left Lebanon by way of Syria in large numbers. France and Italy said they were preparing for a possible evacuation of their citizens.
Hezbollah has shut Lebanon's airport by barricading the road leading to it. The seaport also was closed, leaving one land route to Syria as Lebanon's only link to the outside world.
During the fighting, gunmen burned a newspaper's offices and a Future TV station building belonging to the top Sunni leader, Saad Hariri. They also forced another station to shut down.
Hariri, the son of assassinated former premier Rafik Hariri, and an ally, Druse political leader Walid Jumblatt, holed up in their residences, protected by Lebanese army troops. Opposition leaders said they would not be harmed.
At one point, a rocket-propelled grenade hit the fence of Hariri's heavily protected compound, security officials said. A group of gunmen fired about a dozen bullets at a statue of Rafik Hariri next to the seafront road where he was killed by a truck bomb three years ago.
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Associated Press writers Zeina Karam, Hussein Dakroub and
Scheherezade Faramarzi in Beirut and Matthew Lee in Washington
contributed to this report.
