(Adds eye witness accounts, changes dateline)

TOGLIATTI, Russia, Oct 31 (Reuters) - A bomb on a bus in a south Russian city killed at least eight people and injured 50 during the morning rush hour on Wednesday in what authorities called a terrorist attack.

The blast in the car-making city of Togliatti was probably caused by a bomb hidden under the floor of the bus, local police sources were quoted by Russian news agencies as saying.

Eyewitnesses described how the explosion threw bodies into the air and devastated a peaceful everyday commuting scene.

"The bus went past me. I crossed the crossroads, took a few steps and then I heard this really powerful explosion," a man who lived nearby but did not want to be named told Reuters.

"I saw people who had been thrown out onto the side of the road, I saw corpses. I myself was in a state of shock."

Television pictures showed the green crumpled frame of the bus lying on the road. In a picture taken by an emergency worker, which was obtained by Reuters, a woman could be seen sitting upright but motionless on the wrecked bus.

Other pictures showed the bloody remains of victims, with clothes ripped from bodies -- face down in the debris -- and limbs hanging through blown out windows of the devastated bus.

Apart from workers, the passengers on the bus included students travelling to a local university. Police said the blast was caused by about 2 kg (4.4 lbs) of explosives.

Prosecutors said they had opened a criminal investigation on terrorism charges.

Russia votes in a parliamentary election on Dec. 2. State security officials say they are on alert for any attempts to destabilise the country in the run-up to the vote.

"We have reached the conclusion that this was an act of terrorism," the Samara region governor Vladimir Artyakov said in comments broadcast by Russia's Vesti-24 news channel.

"We are clarifying what type of explosive device was used and we are also clarifying the possibility that there could be more victims."

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An unidentified source quoted by Interfax news agency said Islamic extremists or local organised crime groups could be behind the blast.

But other motives -- including an organised crime feud -- were not being ruled out. Russian officials tend to use the term "terrorism" broadly to describe almost any deliberate attack which causes heavy casualties.

"Eight people are dead, fifty are injured, with ten rescue groups involved. The first arrived within five minutes of the explosion and more soon after," said Vladimir Markhin, a local investigating official.

Prosecutors have also opened criminal cases for murder and the illegal storage of explosives, Russian media reported.

Togliatti, an industrial city on the banks of the Volga river, is more than 1,000 km (600 miles) southeast of Moscow and home to the country's biggest carmaker, AvtoVaz, and is sometimes called Russia's Detroit. The city is named after the late Italian communist leader Palmiro Togliatti.

Russian President Vladimir Putin phoned his envoy to the Volga region and ordered that "every possible measure to give medical assistance to those injured and to help the families of those killed," should be provided, Interfax said.

Organised crime groups, many involved in the trade in car parts, fight regular turf wars and gangland killings are common.

Explosive devices without shrapnel, like the one Russian media said may have caused the bus explosion, are frequently used to settle scores in business feuds.

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