BOSTON bombing suspect Dzhokhar Tsarnaev drove over his dying brother to escape a gunbattle with the police, Watertown's police chief said yesterday.

Dzhokhar and Tamerlan Tsarnaev also hurled a pressure cooker bomb, similar to the ones used in the marathon attack, at officers chasing them in the early hours of Friday, Edward Devau told CNN.

Tamerlan (26) died in the shootout while his brother (19) eluded capture until late Friday, when he was found hiding in a boat in Watertown.

Devau told how Dzhokhar ran over his brother, dragging the body down the street as he escaped from the police who had trapped the older brother.

The suspects were involved in the killing of a police officer at Massachusetts Institute of Technology and then stole a Mercedes-Benz, sparking a chase to Watertown, in the Boston suburbs, according to the police.

Devau said two off-duty policemen were helping four officers who confronted the brothers in the overnight shootout.

"We estimate that over 200 shots were fired over a 10-minute period," Devau said, adding that the brothers also hurled a pressure cooker bomb and other home-made explosives.

"We found a pressure cooker embedded in a car down the street after it caused a major explosion during the gunfight," Devau said.

Two home-made grenades exploded while two others failed to go off.

Another device was found in an abandoned car, he said.

Devau told how Tamerlan suddenly emerged "and just starts walking down the street shooting at police officers trying to get closer".

When the suspected bomber ran out of ammunition, police officers overpowered him and were trying to handcuff him when the other brother drove the stolen Mercedes at them, said the police chief.

"One officer yells 'look out'," said Devau. "They dive out of the way and he runs over his brother and drags him a short distance down the street."

Doctors officially pronounced Tamerlan dead at the hospital, where his brother is now being treated.

Dzhokhar drove off amid more gunfire, but two or three streets away, "he dumps the car and runs into the darkness in the streets.

"Then we lost contact with him," said Devau.

Dzhokhar was eventually cornered. He was wounded and hiding in a boat that was being stored in someone's backyard.

Devau said the suspect made no statement to the police at the scene.