The brown bear was pushing a wheelbarrow during the act in the western Karelia region when it suddenly lunged at the trainer, knocking him to the ground.
The animal can then be seen biting at the man's head and neck as screaming children sit just feet away, with no safety barrier between them and the bear.
The youngsters can be heard screaming as another circus employee kicks the animal in an attempt to free his colleague.
The 600lb beast was prodded with an electric shock device, as the audience fled for the only exit in the travelling circus's big top.
The trainer floored by the bear is understood to be 'injured' but no details were given.
Mother-of-one Galina Gurieva, 27, who filmed the distressing scene in Olonets town, said: 'My knees are still trembling.
'I was shocked there was no fence for the safety of spectators, given the size of the bear.'
Another parent said: 'The spectators rushed outside in panic, including me.'
One more visitor posted: 'It was an enormous bear, and they lost control of him.'
Another said: 'At first, for a few seconds, while recording the video, everyone thought it was a script.
'But when the assistant started kicking the bear and pulled out the shocker, the panic started.
'People got up and ran to the exit.'
Even after the bear was given an electric shock it 'had time to walk in a frenzied state in the arena, thus terrifying the people".
The bear is a star attraction at Anshlag Tent Circus and this performance was billed as 'Bow-legged and the Wheelbarrow'.
A father said: 'We expected to see cutie cubs and they brought in a huge grizzly-terminator.
'They did not protect children and adults from possible attack.'
Police and safety officials are investigating the incident.
Travelling circuses with live animal performances remain popular in Russia despite growing campaigns for bans or curbs on such shows.
Reader Comments