Uzbek soldiers jump out of a truck in downtown Andijon, Friday

Large numbers of people have reportedly been killed in the former Soviet republic of Uzbekistan as government troops seek to put down a rebellion that began Friday. The unrest is the most serious challenge to President Islam Karimov, who's ruled the country since Soviet times.

Clashes resumed Saturday morning in the eastern city of Andijon one day after Uzbek army soldiers violently dispersed civilian protests.

Doctors at a city hospital say at least 50 people were killed in Friday's unrest, with some eyewitnesses saying the death toll may be higher.

A local correspondent for the Russian NTV network reported that around 100 had been killed and 200 wounded, and that medical personnel were arriving in the city.

Numerous eyewitnesses say soldiers fired on thousands of people who had gathered Friday in the main city square to support armed men who freed 23 local businessmen from jail where they have been held since February.

The government says the men belong to an outlawed Islamic organization in a country where only state-controlled religious activity is permitted.

But supporters of the men say the charges are an excuse to clamp down by a government that human rights groups say uses torture and arbitrary arrests against perceived opponents.

Townspeople also say the men run businesses which provide many jobs in the impoverished Ferghana Valley, a region that is known for its anti-government sentiment.

The unrest is the most serious crisis to confront long-time ruler Islam Karimov, who spent part of Friday in the embattled city before returning to the capital, Tashkent.

A similar situation spiraled out of control in neighboring Kyrgyzstan last March and resulted in the downfall of the authoritarian leader there.

In Washington, the White House has called for restraint to avoid further bloodshed, as have various European governments.

Russia has watched events unfold with concern, as the Foreign Ministry blamed the violence on "extremists" and expressed support for Mr. Karimov.

Victor Zavarzin, who heads the Defense Committee in the Russian parliament, says the Uzbek authorities must take control of the situation, and that Russia will limit any involvement according to international law and agreements that exist between the two countries.

Since 2001 Uzbekistan has been host to American troops at an air base for operations in neighboring Afghanistan. The base is located in the south far from the scene of the unrest.