BISHKEK, Kyrgyzstan - Mobs of armed men torched Uzbek neighborhoods in Kyrgyzstan in ethnic clashes Friday that officials said left at least 45 people dead and 637 wounded in the Central Asian nation that hosts U.S. and Russian military bases.

The rioting in Osh, the country's second-largest city, is the heaviest violence since President Kurmanbek Bakiyev was toppled in a bloody uprising in April and fled the country. The unrest also spread to Bishkek, the capital, where armed mobs clashed with police and volunteer militiamen, witnesses said.

The intensity of the conflict, which pits ethnic Kyrgyz against minority Uzbeks, appeared to take authorities by surprise and threw the fragile interim government's prospects for survival into doubt.

Quelling the violence will prove a decisive test of the government's ability to control the country, hold a June 27 vote on a new constitution, and go ahead with October parliamentary elections.

Dozens of buildings across Osh were ablaze Friday after witnesses reported sustained gunfire beginning late Thursday. Gangs of young men with metal bars and stones attacked shops and set cars alight.

The interim government declared a state of emergency in Osh and some nearby areas and dispatched armored vehicles, troops, and helicopters to pacify the situation. Soldiers were posted at routes into the city and at major intersections, but the fighting did not abate. Authorities imposed a curfew from 8 p.m. to 6 a.m. until next Saturday.

Bakyt Omorkulov, a member of the nongovernmental Coalition for Democracy and Civil Society, said he was patrolling the streets with other volunteers to try to prevent further clashes. He said that the troops' presence did not help stabilize the situation and that more buildings were set ablaze as night fell.

"The military are driving around, but it has no effect whatsoever," he said.

He said the streets were filled with young men brandishing sticks and weapons, adding that Uzbek areas were particularly hard hit by the violence.

"Aravan Street is completely destroyed," he said. "Dozens of cafes and buildings are burning - it's the same picture in Cheryomushki. It's like being in Chechnya."

Ikram Abdumalitov, who lives in Osh, said earlier Friday that he saw about 1,000 young and armed Kyrgyz men marching toward Uzbek neighborhoods. "The Uzbeks are in turn chopping down trees and blocking the road to their neighborhood," he said.

Many of the injured had been stabbed or shot, Health Ministry spokeswoman Yelena Bailinova said. A doctor at a hospital in Osh said the death toll could climb sharply because many Uzbeks were too afraid to seek treatment.

Interim President Roza Otunbayeva called for a return to calm in an emotional televised address.

Tensions have long simmered between ethnic Kyrgyz and Uzbek - both Sunni Muslim groups - in Kyrgyzstan's south. In 1990, hundreds were killed in a violent land dispute between the two communities across southern Kyrgyzstan, which borders Uzbekistan.

Kyrgyzstan hosts the Manas U.S. military air base in Bishkek, a crucial support center supplying forces fighting the Taliban in Afghanistan.