LATEST: People are emerging from the centre with their arms raised. Police are yet to locate a shooter or any evidence of shooting. Schools in the area are on lockdown as a precaution.
Reports suggest three shots were fired outside the Naval Medical Centre in Balboa Park, San Diego.
An initial SWAT sweep of the centre showed no signs of injuries of evidence of shooting.
Washington Post reports the centre is 'secured from the inside.'
The reports came from a phone call stating three shots had been fired in the air. Witnesses said they were 'warning shots' but details remain sketchy.
Sailors were told to 'run, hide or fight,' in a Facebook post by the centre.
NBC San Diego has said another naval base, at Point Loma, is also on lockdown.
The SWAT team has arrived and there is a heavy police presence in the area.
Fox 5 said police are currently talking to the shooter and no one has been seriously injured.