VOLGOGRAD, Russia (REUTERS) - Police detained dozens of people on Tuesday in sweeps through the Russian city of Volgograd after two deadly attacks in less than 24 hours that raised security fears ahead of the Winter Olympics.

A man wounded when a bomber set off a blast in the city's railway station on Sunday died overnight, bringing the toll in that attack to 18. Investigators said 14 people died in a bus bombing on Monday but health officials put the toll at 16.

There was no indication that any of those arrested was connected to the attacks.

Nobody has claimed responsibility for the blasts, but they underscored vulnerability to bombings and raised fears of attacks by Islamist insurgents whose leader has called on militants to prevent Russia hosting the Olympics in February.

Mourners laid flowers at the site of the suicide bombing that tore the bus apart.

"I'm frightened," said Tatyana Volchanskaya, a student in Volgograd, 700km north-west of Sochi, the Black Sea resort where the Winter Games are to start on Feb 7. She said some friends were afraid to go to shops and crowded places.

The attacks posed a challenge to President Vladimir Putin, who oversaw a war that drove rebels from power in Chechnya over a decade ago but has been unable to quell the Islamist insurgency that erupted in its wake.

Volgograd - formerly Stalingrad - is a city of about 1 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.

Two people were killed in a bomb blast in Dagestan 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, about 5,200 police and interior troops were mobilised in "Operation Anti-terror Whirlwind", the head on a emergency headquarters, 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, authorities initially described the bomber as a woman from Dagestan, a hub of Islamist militancy in the Caucasus, but later said the bomber 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 Mr 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.