Emergency services and ambulances raced to the scene of the suspected terror attack in the city of Kerch, which is the entry point for a new bridge linking Crimea to mainland Russia.
Casualties have been rushed to nearby hospitals, many with serious injuries.
The Russian national anti-terrorism committee said the blast was caused by an “unidentified explosive device”.
Witnesses suggested there may have been gunfire and attempts to take hostages prior to the explosion which ripped through the polytechnic’s cafeteria.
Police have not commented on the blast and Russian media initially reported a gas explosion.
The suspected terror attack will attract intense scrutiny from Russian authorities.
Crimea was annexed by Russia from Ukraine in 2014 and providing security to the peninsula has been a priority for Moscow.