BISHKEK, Kyrgyzstan (AP) — Mobs of armed men torched Uzbek neighborhoods in Kyrgyzstan on Friday in ethnic clashes that officials said left at least 45 people dead and 637 wounded in a 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 former President Kurmanbek Bakiyev was toppled in a bloody uprising in April and fled the country. The unrest also spread to the capital, where armed mobs clashed with police and volunteer militia, according to witnesses.

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 new parliamentary elections scheduled for October.

Dozens of buildings across Osh were ablaze Friday after witnesses reported sustained gunfire beginning late Thursday. Gangs of young men armed 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 June 20.

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

"We don't feel the authorities' presence," he told The Associated Press in a telephone interview. "The military are driving around, but it has no effect whatsoever."

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, dozens of cafes and buildings are burning — it's the same picture in Cheryomushki. It's like being in Chechnya," he told the AP.

Ikram Abdumalitov, who lives in Osh, said earlier in the day that he saw about 1,000 young and armed Kyrgyz men marching toward Uzbek neighborhoods in eastern Osh.

"The Uzbeks are in turn chopping down trees and blocking the road to their neighborhood," Abdumalitov said.

Armed men flooded in from nearby villages to join the fight, a trader in Osh said on condition of anonymity due to the volatile situation.

Many of the injured had been stabbed or shot, Health Ministry spokeswoman Yelena Bailinova said, as she gave the death toll. She said many of those wounded were in grave condition.

A doctor at a hospital in Osh said the death toll could climb sharply because many Uzbeks were too afraid to seek treatment.

"All the beds in this hospital are full, but 90 percent of the people being treated are Kyrgyz, because Uzbeks are afraid of the Kyrgyz victims' relatives, who are in an extremely aggressive frame of mind," the doctor said. He spoke on condition on anonymity, as he was not authorized to speak to the media.

Smaller-scale ethnic violence also broke out late Friday in the capital, Bishkek, where a mob of Kyrgyz men attacked and robbed ethnic Uzbeks at a popular bazaar. As night fell, the crowd swelled and clashes with police erupted.

Witnesses said busloads of Interior Ministry troops were driven to the market in an attempt to disperse the mob, but they left the scene after a tense and violent standoff.

In another part of the city, witnesses saw an armed mob exchanging gunfire with volunteer militia who tried to maintain order.

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

"I would like to appeal in particular to the women of Kyrgyzstan. Dear sisters, find the right words for your sons, husbands and brothers. In the current situation, it is unacceptable to indulge in feelings of revenge and anger," she said.

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.

At a security summit in Uzbekistan, Chinese President Hu Jintao and Russian President Dmitry Medvedev both expressed concern over Friday's fighting and promised to help Kyrgyzstan, a former Soviet republic of 5 million people, restore order.

"We are really interested in seeing Kyrgyzstan overcome the stage of internal upheaval as quickly as possible and solve the task of forming a modern government capable of tackling acute problems of socio-economic development," Medvedev said.

U.N. Secretary-General Ban Ki-moon expressed deep concern at the renewed violence and called for calm to be restored, the rule of law to be respected, and all issues to be resolved peacefully through dialogue, U.N. associate spokesman Farhan Haq said. Ban also urged the interim government "to pay particular attention to interethnic relations in the country."

Bakiyev is believed to be in exile in Belarus, but interim authorities accuse his supporters of trying to foment unrest to undermine their control and derail the upcoming referendum and parliamentary election.

Kyrgyzstan also hosts the Manas U.S. military air base in Bishkek, a crucial support center supplying forces fighting the Taliban in Afghanistan. Bakiyev's government had vowed to close the base last year, but later agreed to let U.S. forces stay after raising the rent to $63 million from $17 million.

In recent weeks, operations at Manas have been hindered by a dispute over the interim government's decision to tax fuel sold to the base. The U.S. military says it has stopped refueling tanker planes at Manas while fuel prices are being renegotiated, but flights to ferry military personnel and supplies to and from Afghanistan have continued.

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Peter Leonard reported from Almaty, Kazakhstan.