The walls, floors and roof of a St Petersburg university building partly collapsed on Saturday trapping two dozen people under debris, Russian news agencies said, citing emergency services.
There were no casualties and no immediate injuries were reported, they added.
At least 80 were evacuated from the building.
Russia's Emergency Ministry said the incident occured while renovations were taking place at the National Research University of Information Technologies, Mechanics and Optics (ITMO), but the cause of the partial collapse was not known at the time of writing.
Acting St Petersburg governor Alexander Beglov had gone to the scene, TASS news agency reported.
The university building is located in the centre of Russia's second-biggest city.