A video grab shows a bus damaged by an explosion on a street in the city of Togliatti, October 31, 2007. [Photo: Reuters TV/Reuters]

The death toll from a bus explosion in the southern Russian city of Togliatti rose to at least eight, news agencies reported on Wednesday.

The number of injured increased to 45 from 20 reported earlier.

The explosion occurred in the early morning in the Volga river city of Togliatti in Samara region.

"A criminal case has been launched to investigate the explosion as a terrorist attack," said Samara regional governor Vladimir Artyakov.

A group of experts who have investigated terrorist attacks on the public transport system in Moscow will soon leave for Togliatti to help probe the blast, the Interfax news agency reported.

Police said the explosion was caused by a bomb, which was equivalent to 300 grams to 1 kg of TNT.

"There are reasons to believe that the bomb was planted either on the underside of the bus or on its floor," a law enforcement official was quoted by Interfax as saying.

"Probably, one of the passengers was carrying it, or it was laid, when the bus was on the way," said a police spokesman.

Four of the injured were in serious conditions and three children were among those hospitalized, a source with Togliatti's central ambulance center said.

Four people were hospitalized with burns while others sustained injuries, bruises and cuts, the source said.