* Trolleybus bombing follows Sunday's railway station attack
* Bombings underscore Russia's vulnerability ahead of Sochi Games
* Explosion guts bus, leaves bodies in street
* More attacks can be expected, says anti-terrorism veteran (Adds details, quotes, context)
By Maria Tsvetkova
VOLGOGRAD, Russia, Dec 30 (Reuters) - A bomb blast ripped a trolleybus apart in Volgograd on Monday, killing at least 10 people in the second deadly attack in the southern city in two days and raising fears of further violence as Russia prepares to host the Winter Olympics.
The morning rush-hour bombing, which left mangled bodies in the street, underscored Russia's vulnerability to militant attacks less than six weeks before the Sochi 2014 Games, a prestige project for President Vladimir Putin.
It came less than 24 hours after a suicide bomb blast killed at least 17 people in the main railway station in the same city, a major transport hub in southern Russia.
A Reuters journalist saw the blue-and-white trolleybus reduced to a twisted, gutted carcass, its roof blown off and bodies and debris strewn across the street. Federal investigators called the blast a "terrorist act".
"For the second day, we are dying - it's a nightmare," a woman near the scene said, her voice trembling as she choked back tears. "What are we supposed to do, just walk now?"
The consecutive attacks will raise fears of a concerted campaign of violence before the Olympics, which start on Feb. 7 in Sochi, about 430 miles (690 km) southwest of Volgograd.
In a video posted on the web in July, the leader of insurgents who want to carve an Islamic state out of the North Caucasus, a string of Muslim provinces south of Volgograd, urged militants to use "maximum force" to prevent the games from being held.
A female suicide bomber from the North Caucasus killed six people on a bus in Volgograd in October.
Spokespeople for the International Olympic Committee in Lausanne, Switzerland, could not immediately be reached for comment on the most recent bombings.
BIGGEST SECURITY CHALLENGE
In power since 2000, Putin secured the Games for Russia and has staked his reputation on a safe and successful event. He has law enforcement agencies to ensure security at the Olympics in Sochi, at the northwest end of the Caucasus mountain range.
Putin was first elected after winning popularity for a war against Chechen rebels, but attacks by Islamist militants whose insurgency is rooted in the war have clouded his 14 years in power and now confront him with his biggest security challenge.
There was no immediate claim of responsibility for either of the Volgograd attacks, which left dozens wounded in addition to the deaths. Authorities said 37 people were hospitalised after Sunday's attack and 23 were reported wounded on Monday.
After the attack on Sunday, in which authorities said a bomber detonated explosives in front of a metal detector at Volgograd station, Putin ordered law enforcement agencies to take measures to enhance security.
Police said additional officers would be sent to railway stations and airports nationwide and more extensive security checks would be conducted.
But the attacks raised questions about the effectiveness of security measures that the Kremlin routinely orders increased after bombings.
More attacks can be expected before the Olympics and cities in southern Russia where the Games are not being held are easier targets than Sochi, said Alexei Filatov, deputy head of the veterans' association of the elite Alfa anti-terrorism unit.
"The threat is greatest now because it is when terrorists can make the biggest impression," he told Reuters. "The security measures were beefed up long ago around Sochi, so terrorists will strike instead in these nearby cities like Volgograd."
"Half of our traffic police and emergency workers have been sent to Sochi," said Alexei, a Volgograd resident whose wife called from her car after hearing the blast and seeing the gutted trolleybus.
"She heard cries - I could hear her voice shaking," he said. "Then she locked herself in the car and gave way to hysterics."
A city of about 1 million, Volgograd is one of the venues for the 2018 soccer World Cup, another high-profile sports event Putin has helped Russia win the right to stage, and which will bring thousands of foreign fans to cities around Russia.
Sunday's attack was the deadliest to strike Russia's heartland since January 2011, when a male suicide bomber from the North Caucasus killed 37 people in the arrivals hall of a busy Moscow airport.
Investigators initially said a woman set off the bomb that filled the front of the railway station building with a massive orange fireball on Sunday, but later said the bomber could have been a man. (Additional reporting by Alissa de Carbonnel and Lidia Kelly in Moscow; Writing by Steve Gutterman; Editing by John Stonestreet)