* More than 40 pro-Russian Ukrainians died in fire
* Pro-Kiev football supporters say were ambushed
* Protest camp torched but cause of building fire unclear
By Elizabeth Piper
KIEV, May 3 (Reuters) - Passions ran high in Odessa before local soccer team Chornomorets was due to play Kharkiv's Metalist.
Fans who joined together to sing support for Ukraine's new leaders knew they might attract some trouble in the Russian-speaking city, but no one expected the chain of events that ended in the death of over 40 people.
"Everything that happened was like in a horror movie," said Nadiya Yashan, a Chornomorets supporter who accused pro-Russian militants among the away fans of launching a ferocious attack.
"We never expected an ambush on such a scale and the police to do so little," she told Ukrainian television.
Over a few hours, running battles moved through the city, along the streets to a camp occupied by pro-Russian supporters, then to a Soviet-era trade union building where dozens of rebels disdainful of rule from Kiev later died in a blaze that engulfed the building.
The mood on a warm afternoon had been friendly. Some supporters of both teams joined together to walk the route to the stadium before kickoff, displaying their support for the new pro-Western authorities, chanting their contempt for Russian President Vladimir Putin.
After President Viktor Yanukovich was toppled in an uprising in February, some Ukrainian football teams' supporters have started to put aside sporting rivalries before games to show support for Ukrainian unity, often singing the national anthem and anti-Putin songs.
According to local news website www.viknaodessa.od.ua, the starting point of the fighting was when the two teams' supporters were walking down Greek street. There armed men, wearing masks, in black attacked them, some shooting pistols.
Television pictures showed police with shields attempting to form a barrier between the fans and pro-Russian activists
The fans responded by throwing stones. Many tried to flee.
Why the battles moved from the street grid of Odessa's mathematically laid out city is not entirely clear.
Many say Kiev supporters heard that shots had been fired at a square called Kulikovo Polye, or Kulikovo field, and ran there to find out what had happened. Others say they went there to torch the tent city that pro-Russian supporters had built in protest against Kiev.
Tents were burnt and some pro-Russian supporters decided to hide in the nearest building, the news website said. That was the stately, white five-storey trade union building.
"Who let them in and why? I just don't know. That's not a question for me," said one Odessa native.
It is not clear who first started throwing petrol bombs.
At least one town official suggested the fire may have started on the third floor, which would suggest that the cause of the blaze was something other than a petrol bomb thrown from outside. That could not be independently confirmed.
"The fire engines came very late. The fire was raging. Those who hadn't been overcome by smoke, just started jumping out of windows. It's a tall building so you can imagine what happened," said an eyewitness.
Some talk of a "third force", a reference to what Kiev calls pro-Russian "saboteurs" accused of fomenting unrest in Ukraine to stop it travelling a pro-Western path and keep it firmly within the Kremlin's embrace.
Moscow denies playing any role in the uprisings in Ukraine and has said Kiev and its Western backers were responsible for the deaths in Odessa.
Witnesses say both sides tried to help those trapped in the building. Some carried scaffolding to windows to try to get people out.
The charred building is now surrounded by flowers and hundreds of people calling for Russia to help.
Announcement of a police investigation is unlikely to ease anger. Pro-Russians have little confidence in the police while Kiev has often only a tentative hold on its security forces in many cities.
"I grieve with Odessa," said Anatoly Shanin on the social media site VKontakte. "It is difficult and painful to see your hometown in flames." (Additional reporting by Natalia Zinets and Yvonne Bell; Editing by Ralph Boulton and Robin Pomeroy)