SWAT teams are in a tense stand-off with a gunman after a gang of men opened fire with automatic weapons outside a police headquarters in the US.

At least one of the gunmen fled in an armoured van, leading a police chase through the streets of Dallas during which the vehicle rammed a police cruiser.

Scenes of the chase were caught on camera by members of the public. At one point the armoured vehicle is seen driving straight at a police officer's car under a hail of gunfire. The officers are pictured fleeing from the scene.

The vehicle was finally surrounded in a car park and specialist officers are in telephone negotiations with one of the suspects inside.

Officers said the man has identified himself as James Boulware and is said to have a history of domestic violence. Police are understood to fear Boulware could be carrying further explosives in his vehicle.

They have blocked off the entrance to the City Hall underground car park.

Dallas police said they had discovered a pipe bomb in one of four bags outside the Dallas Police Department headquarters and they have now increased security at all police facilities. A controlled explosion has been carried out.

Dallas Police Chief David Brown said the shoot-out happened just after midnight when the men pulled up to the building and started firing.

He said as many as four suspects were involved in the attack - some of whom were firing from higher vantage points.

Dallas Police Major Max Geron said he was not aware of any injuries related to the shooting but added that the police had activated the Emergency Operations Centre to coordinate the response.

He said: "As a precautionary measure, we've taken additional steps to ensure security at all police facilities."