At least eight people have been killed and more than 50 injured in an explosion on a bus in the southern Russian city of Togliatti.
The explosion happened during the morning rush hour
The governor of the Samara region said the attack was being treated as a "terrorist" act.
Security officials say an explosive device may have been fixed to the vehicle, or carried on board.
It was the second blast on a bus in Russia in just over a week, following an explosion in the Dagestan region.
The explosion happened shortly after 0800 (0400 GMT) on Wednesday, as many people were making their way to work in the industrial city on the Volga river.
A rescue service spokesman said the powerful blast destroyed the glass in the windows of a nearby building.
The injured are receiving treatment for burns and shrapnel wounds.
Russian news agencies quote a police source as saying explosives were either planted underneath the bus or on its floor.
Authorities are also investigating whether a passenger may have been carrying a bomb.
A team of anti-terrorist experts is flying from Moscow to help in the investigation.
Togliatti is the headquarters of AvtoVAZ, the country's largest carmaker.
The city has a reputation for gang violence between groups vying for control of the car business.