VOLGOGRAD, Russia (Reuters) - Russian President Vladimir Putin on Tuesday vowed to annihilate "terrorists" following two deadly bomb attacks in less than 24 hours in the southern Russian city of Volgograd that raised security fears ahead of the Winter Olympics.
The uncompromising remarks in a New Year's Eve address were Mr Putin's first public comments since suicide bombers killed at least 34 people in attacks on a railway station and a trolleybus on Sunday and Monday.
The bombings raised fears of further attacks before Russia hosts the Winter Olympics in less than six weeks in the Black Sea resort of Sochi, a major prestige project for Mr Putin.
"I am certain that we will fiercely and consistently continue the fight against terrorists until their complete annihilation," Mr Putin said, according to Russian news agencies.
"Dear friends, we bow our heads before the victims of cruel terrorist acts," the Russian leader said on a visit to the east coast city of Khabarovsk.
Mr Putin, who first became president when his predecessor Boris Yeltsin stepped down and named him to the post exactly 14 years ago on Tuesday, has been unable to crush Islamist militants in the Muslim provinces of the North Caucasus.
Police on Tuesday detained dozens of people in sweeps through Volgograd, although there was no indication that any of them was connected to the attacks, for which nobody has claimed responsibility.
Mourners laid flowers at the site of the bombing that tore the bus apart and left residents fearing further violence.
"I'm frightened," said Ms Tatyana Volchanskaya, a student in Volgograd, 700km north-west of Sochi, where the Games start on Feb 7. She said some friends were afraid to go to shops and other crowded places.
Volgograd - formerly Stalingrad - is a city of about one million and a transport hub for an area of southern Russia that includes Chechnya and the other mostly Muslim provinces of the North Caucasus, where the insurgency generates deadly violence almost every day.
A car bomb killed a prosecutor's assistant in Dagestan, a hub of Islamist militancy in the Caucasus, on Tuesday, and two people were killed in a bomb blast there late on Monday, authorities said.
Mr Putin has staked his prestige on the Games in Sochi, which lies at the Western edge of the Caucasus Mountains and within the strip of land the insurgents want to carve out of Russia and turn into an Islamic State.
He ordered increased security nationwide after the attacks, the deadliest outside the North Caucasus since a suicide bomber from a province next to Chechnya killed 37 people at a Moscow airport in January 2011.
In Volgograd, more than 5,000 police and interior troops were mobilised in "Operation Anti-terror Whirlwind", Interior Ministry spokesman Andrei Pilipchuk, said on state TV.
He said 87 people had been detained after they resisted police or could not produce proper ID or registration documents, and that some had weapons. State TV showed helmeted officers pushing men up against a wall. But there was no sign any were linked to the bombings or suspected of planning further attacks.
The Itar-Tass news agency said police were focusing on migrant workers from the Caucasus and ex-Soviet states - groups that rights activists say face prejudice and are often targeted by police indiscriminately.
Investigators said they believed a male suicide bomber was responsible for Monday's morning rush-hour blast, which turned a trolleybus into a twisted wreck and left bodies lying in the street.
In Sunday's attack on the station, authorities initially described the bomber as a woman from Dagestan, but later said the perpetrator may have been a man.
Citing unnamed sources, the Interfax news agency said the suspected attacker in Sunday's blast was an ethnic Russian convert to Islam who had moved to Dagestan and joined militants there early in 2012.
Volgograd was also the scene of an attack in October, when a woman from Dagestan killed seven people in a suicide bus blast.
The violence raised fears of a concerted campaign before the Olympics, an important project for Mr Putin, who secured Russia's first post-Soviet Games in 2007, during his initial 2000-2008 stint as president.
Intended to showcase how Russia has changed since the collapse of Soviet communism in 1991, the Games have also been a focus for complaints in the West and among Russian liberals that Mr Putin has stifled dissent and encouraged intolerance.
This month, Mr Putin freed jailed opponents including oil tycoon Mikhail Khodorkovsky and the Pussy Riot punk band in what critics said was an effort to disarm Western criticism and improve his image.
In an online video posted in July, the Chechen leader of insurgents who want to carve an Islamic state out of mainly Muslim provinces south of Volgograd, urged militants to use"maximum force" to prevent the Games from going ahead.
International Olympic Committee President Thomas Bach condemned the "despicable attack on innocent people" and said he had written to Putin to express condolences and confidence that Russia would deliver "safe and secure games in Sochi".
The US government is concerned Islamist militants may be preparing attacks aimed at disrupting the Olympics and has offered closer cooperation with Russia on security.