A suicide bombing on a packed-trolley bus in Volgograd killed 17 people and wounded dozens of others.
The city was on high alert on Monday following the latest attack, which came a day after a suicide bombing at a train station there killed 17 people.
The blasts have raised safety concerns in the region in the run-up to the 2014 Winter Olympics due to take place in February in Sochi, about 650km southwest of Volgograd.
Volgograd, a city of a million people on the west bank of the River Volga, serves as a gateway to the southern wedge of Russian territory bounded by the Black and Caspian Seas and the Caucasus mountains.
"The explosives were detonated by a male suicide bomber, fragments of whose body have been found and taken for genetic analysis to establish his identity," Vladimir Markin, spokesman for Russia's federal Investigative Committee, said.
The force of Monday's blast destroyed the number 15A trolley-bus, which was packed with early morning commuters at about 8:30am local time (04:30 GMT).
Markin said the suicide bomber set off 4kg of explosives, and noted that the explosives were identical to those used in Sunday's train-station bombing.
"This confirms the theory that the two attacks are linked. It is possible that they were prepared in the same place," he said.
Russia's Foreign Ministry called for international solidarity in the fight against "terrorism" on Monday, though it didn't place direct blame on any group for the bombings.
|Fred Weir puts Volgograd attacks in context
"We will not retreat and will continue our consistent fight against an insidious enemy that can only be defeated together," the ministry said in a statement.
Russian investigators have opened a criminal probe into a suspected act of terror as well as the illegal carrying of weapons, Markin told Interfax.
Russian President Vladimir Putin has sent Alexander Bordnikov, director of Federal Security Service, to Volgograd while the Interior Ministry raised the number of its staff in the city.
The ministry was also expected to send a plane to Volgograd for transporting the wounded in Monday's attack to Moscow for medical treatment, according to Interfax.
The attack was the third on the city in the past three months.
On October 21, a female suicide bomber blew herself up on a bus, killing six people.
"The people are angry. They feel let down by the social services, by the police, by the security forces," Al Jazeera's Peter Sharp, reporting from Volgograd on Monday, said.
"They feel not enough has been done to protect them, to protect their families.
"About 400 local people gathered to protest about what's happened here over the last 24 hours, and the police cleared them off, saying their presence altogether en masse was just another target for the people who are carrying out these bombings."
Threat to Winter Olympics
Russian officials said security would be stepped up in Moscow before New Year, the biggest holiday of the year in the country, which is traditionally marked by mass outdoor gatherings.
Reports said Russia's second city of Saint Petersburg had already cancelled its planned New Year fireworks display.
There was no immediate claim of responsibility for either of the Volgograd attacks, which came several months after Doku Umarov, a Chechen rebel leader, called for new attacks against civilian targets in Russia, including Sochi Games.
In the past, armed groups have carried out attacks in cities beyond the borders of the North Caucasus.
Rebels have said they want to carve out a Muslim state in the North Caucasus. Their armed campaign is rooted in two post-Soviet wars in Chechnya, one of the region's provinces.
Putin, who was first elected president in 2000 after launching the second war in Chechnya, which drove Chechen separatists from power, has staked his reputation on a safe and successful Winter Games.