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At least 10 people have been killed and more than 20 others hurt in a suspected suicide bombing on a trolleybus in the Russian city of Volgograd.
The blast comes a day after 17 people died in another suicide attack at the central station in the city.
Security has been tightened at railway stations and airports across Russia.
Moscow is concerned militant groups could be ramping up violence in the run-up to the 2014 winter Olympic Games in the city of Sochi.
The Olympic venue is close to Russia's volatile north Caucasus region.
Through the day, officials issued conflicting statements on casualties. Officials said that the suspected bomber was a woman, but late Sunday the Interfax news agency quoted an unidentified law enforcement officer as saying footage taken by surveillance cameras indicated that the bomber was a man. There was no immediate confirmation of that claim from any official sources.
The bomber detonated explosives in front of a metal detector just beyond the station's main entrance when a police sergeant became suspicious and rushed forward to check ID, officials said. The officer was killed by the blast, and several other policemen were wounded.
"When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device," Vladimir Markin, the spokesman for the nation's top investigative agency, said in a statement. He added that the bomb contained about 10 kilograms (22 pounds) of TNT and was rigged with shrapnel.
