Russian bomb technicians began scouring the wreckage of a Volgograd trolley bus with some of the dead still lying at the scene.
At least 10 people were killed and around 23 wounded in Monday’s bomb blast, the second in as many days.
On Sunday at least 17 people lost their lives in a suicide attack at the city’s main railway station.
The blasts have come just weeks before the 2014 Winter Olympics in Sochi.
There was no immediate claim of responsibility for either.
But an internet video posted in July by the leader of insurgents seeking an Islamist state in southern Russia urged militants to stop the games going ahead.
Police say extra officers are being sent to railway stations and airports nation-wide, and will conduct more extensive security checks.