Update: 15.57 Update from the press conference led by Chief Inspector Carol Trevithick:
Five people were injured, three contractors, one police staff (civilian) and one police officer (smoke inhalation). Three have burns injuries with the one most seriously injured taken by air ambulance to Broomfield in Chelmsford. The post title has been updated to reflect this.
Contractors were working in the firing range next to the armoury in the basement of the police station when the explosion happened. The cause is being investigated.
Prisoners have been securely transferred to West Norwood police station.
Update and photos from the scene by Andy Thornley.
Update 14:33 Reports now suggest that seven people have been injured in the blast, one seriously. Five people have been taken to hospital. The title of this post has been updated to reflect the development.
Four people were injured in an explosion at Croydon police station this morning. Police have confirmed that the four are all builders who were working on the site. The explosion is under investigation, but is not thought to be related to terrorism.
Eyewitnesses report around 20 emergency vehicles at the site, including London's Air Ambulance. One man has been airlifted to the burns unit at Broomfield Hospital in Chelmsford.
The police station has been evacuated and nearby Park Lane south-bound and Friends Road are closed.
Croydon police tweeted: "We would like to reiterate though that we’re still operational despite the incident at Croydon police station".
The Croydon Guardian is updating a live blog of the situation, including images.