Protesters angry over the killing of an opposition leader set fire to Albania's main government building Sunday and sent the prime minister and his Cabinet fleeing in a hail of gunfire.

One protester was killed and four of Prime Minister Fatos Nano's guards were wounded as marchers and police traded gunfire. Clouds of smoke from burning cars rose over the center of the capital.Opposition leader Sali Berisha urged his followers to remain calm until Monday's funeral of popular politician Azem Hajdari, 35, who was shot to death Saturday in front of opposition Democratic Party headquarters. A bodyguard was also killed.

Berisha blamed Nano's Socialist Party for the killing and warned him to resign within 24 hours or face "catastrophic consequences." Berisha's followers, some of them weeping in grief, chanted "kill, kill, kill Fatos Nano."

Elsewhere, witnesses said Berisha's followers blocked the national highway with barricades and flaming tires and seized the police station at Kavaja, 18 miles southwest of Tirana.

The violence raised fears of a repeat of the 1997 nationwide riots that swept this country after the collapse of pyramid investment schemes that cost many Albanians their life savings.

Some fear instability in Albania could spread throughout the southern Balkans and impede U.S. and European efforts to solve the crisis in neighboring Kosovo, where ethnic Albanians are fighting for independence from Serbia, the main republic of Yugoslavia. The rebels maintain sanctuaries in northern Albania.

The U.S. Embassy in Tirana urged Albanians to "put down their weapons, cease threatening violence and pursue legitimate democratic and legal means to deal with the situation."

"Leaders of all Albania's political parties must behave responsibly," the American statement said.

Late Sunday, supporters brought Hajdari's body to the sprawling Skanderbeg Square for an all-night vigil before Monday's funeral. President Rexhep Meidani called on police to find Hajdari's killers as quickly as possible.

The Interior Ministry announced a $50,000 to $100,000 reward for information leading to the arrest of the assassins.

There was no immediate claim of responsibility.

On Sunday, Berisha warned the opposition would use "all means" to force Nano out and blamed his party for the killing.

After Berisha issued his ultimatum, about 1,500 of his followers marched to the Interior Ministry, firing pistols in the air and pelting the building with stones.

The crowd set fire to a ministry vehicle, then continued down the city's main street to the prime minister's office. Protesters opened fire on the third floor, which contains Nano's offices. Troops responded with volleys of gunfire over the protesters heads.

But the crowd forced its way into the building, traded gunfire with the guards and set fire to the ground floor. Nano and his Cabinet were holding an emergency meeting at the time.

They fled out a side exit through the smoke and amid explosions of gas tanks of seven cars that the protesters had set on fire.

Later Sunday, the government announced it would take "all measures necessary" to defend the state against those "calling for revenge and blood."