Dust-covered students have been filmed stumbling through rubble still in shock after a Russian university building has collapsed trapping an estimated 21 people.

Security services said more than 80 people have been evacuated after the fifth and sixth floors of the National Research University of Information Technologies, Mechanics and Optics in St. Petersburg collapsed earlier today.

Students were filmed clambering over ruins inside the smoke-filled building and could be heard coughing as they inhaled the dust.

Pictures from social media show the roof has caved in of the university building, from which 60 people have been evacuated

A rescue worker with a service dog in the building where the floors between the first and the fourth storeys have collapsed

One source claimed 150 square metres of the building have collapsed, but the cause is not yet clear. News reports said there may have been students in the building for elective weekend instruction.

According to Russian agencies that several people were trapped under debris in the building but there was no immediate information on casualties.

The roof and several floors of the building collapsed during renovation work. TASS news agency said that acting St Petersburg governor Alexander Beglov had gone to the scene.

The workers rummage through the rubble to look for anyone who may be trapped inside

Russian Emergency employees work at the scene of the collapse building of the National Research University of Information Technologies, Mechanics and Optics in St. Petersburg, Russia

Fire fighters and rescuers work wait anxiously near the St. Petersburg Research University building

Student Fedor Gusak was on the top floor and said: 'The whole building trembled, shaking, and I thought that everything was falling apart.

'The electricity went off. 'It was very scary at this moment. I opened the door to the corridor to see what had happened.

'There was unreal dust, I saw a hole in the roof and no stairs to go down. No exit. While we were thinking what to do, people shouted 'Is there anyone alive?'.

'The firemen arrived very quickly and we descended the fire ladders. There was really no panic, everyone reacted very calmly.'

This picture from social media taken from the opposite side of the street shows how the roof of the university building has caved in

Around 21 people are thought to be trapped inside the building while 60 others have been evacuated

A Russian Emergency employee tries to get inside the collapsed building of the ITMO university

Emercom workers outside the building of St Petersburg's ITMO University where the floors between the first and the fourth storeys have collapsed

St Petersburg Acting Governor Alexander Beglov (right) helps a woman get into an ambulance vehicle outside ITMO university