The injured were rushed to the Colombo National Hospital, they said.
Two of them had suffered serious injuries, Dr Anil Jasinghe, hospital director, said.
Rioting inmates threw debris on to the main Baseline Road highway from the roof of the prison. Baseline Road was closed for traffic as the riot intensified.
Although reasons for the riot was not clear, sources said the inmates were angered when jail authorities launched a search for drugs in the prison.
The military and the police special task force were deployed to deal with the situation.
A similar riot at Welikada in 2010 sent over 40 police and prison guards to hospital.