Casualties are feared, Navy sources said.
The explosion resulted in a major fire breaking out on board INS Sindhurakshak, a Russian-made Kilo class submarine of the Indian Navy, shortly after midnight, they said.
The fate of 18 persons on board the 2,300 tonne submarine, powered by a combination of diesel generators and electric batteries, is being ascertained, a defence spokesperson said. The Navy has ordered a board of inquiry to probe the explosion and subsequent fire in the submarine, he said.
However, due to the explosion, the submarine has submerged at the dock with only a portion visible above the surface, a defence statement said.
TV footage of the incident showed a huge ball of fire triggered by the explosion lighting up the night sky in Colaba area where the Navy dockyard is located.
The statement said efforts are on to ascertain the safety of the personnel and salvage the submarine.
Navy Chief Admiral D K Joshi is on his way to Mumbai.
INS Sindhurakshak was not on active duty at the time of the accident, Navy sources said.
The incident has come at a time when the Navy is faced with a depleting submarine fleet.
Commodore (retd) Uday Bhaskar, a former IDSA director, said since the rate of induction of new platforms has not kept up with the kind of wear and tear that a submarine would undertake, the net result is that the Navy's submarine fleet is depleting and the operation load is increasing.
