BEIRUT, Lebanon -- Heavily armed Hezbollah fighters seized control of much of western Beirut on Friday, patrolling the deserted streets in a raw show of force that underscored the Shiite militia's refusal to back down in its escalating confrontation with the American-backed government.

Hezbollah allies also forced a government-allied satellite television station off the air and burned the offices of its newspaper affiliate, as Sunni fighters loyal to the government largely melted away after three days of the worst sectarian clashes Lebanon has seen since its 15-year civil war.

Those humiliating blows made clearer than ever the power and determination of Hezbollah, backed by Iran and Syria, and its allies. By Friday afternoon, a long column of armed Shiite fighters was riding joyfully through west Beirut in trucks, cars, and scooters, shouting and firing their weapons into the air in a raucous victory celebration.

The government majority issued an urgent appeal for help from other nations on Friday evening, calling Hezbollah's actions an "armed coup" against Lebanon and its democratic system using "weapons sent by Tehran." Some government lawmakers, including the Druse leader Walid Jumblatt and Saad Hariri, the son of former Prime Minister Rafiq Hariri, spent the day holed up in their compounds, protected by Lebanese Army contingents and police.