Several shots have been fired from an armored vehicle at the police headquarters in Dallas on Saturday morning, police say.

The vehicle drove off, pursued by dozens of police cars, and is now involved in a stand-off in a car park, officials said.

Police said officers responded to a call for an active shooter at the police headquarters at 12:27 AM. Shots were fired at the building, police said.

There has been an exchange of gunfire in the car park, police added.

A bag left outside the police station was later confirmed to contain explosives including pipe bombs, police said on Twitter.

A possible suspect has been identified as James Boulware, police said.

The motive for the shooting is not yet clear.

Dallas police say no police officers have been injured.