There were no immediate reports of casualties at the Salawa military camp where security forces are known to store some of their heavy weaponry and ammunition, including rockets, police said.
Residents in Salawa, 36 kilometres (26 miles) east of Colombo, were seen leaving their homes as police reported shrapnel hitting the main highway.
The night sky was lit with an orange glow and the area shook at regular intervals. Flying debris could be seen from three kilometres away, an AFP photographer said.
Firefighters were stopped from reaching the camp as it was unsafe for them to get close to the source of the explosions. The cause of the blasts was not yet known, said the police officer who asked not to be named as he is unauthorised to speak to the media.
Also Read
The explosion was the worst at a military installation since the end of Sri Lanka's decades-long Tamil separatist war in May 2009.
In June 2009, there was a similar, but a much less intense explosion at an army ammunition dump in the northern district of Vavuniya, 250 kilometres north of Colombo, leaving several soldiers injured.