9.47am BST

The police officer said they pursued the suspects into a residential neighbourhood in Watertown, where there was an exchange of gunfire with the suspects, who were in an SUV.

During the course of the pursuit, several explosive devices were discharged from the car towards officers, he said.

During the exchange of gunfire, one of the suspects was "struck" and taken into custody.

A second suspect fled from the car and is being sought at the moment.

A police officer was also struck and is in hospital in critical condition.

Police from Boston, Massachusetts and Watertown are seeking the other suspect.

He told residents not to leave their homes. If they see something suspicious, they should call 911 immediately.

He warns motorists not to stop for anyone or pick up anyone at the side of the road.

He said they are looking for "Suspect No 2, the white-capped individual who was involved in Monday's bombing of the Boston Marathon".

He is dressed in a grey, hoodie-type sweatshirt. He is a light-skinned, Caucasian male with longer brown curly hair.

Suspect No 1 was shot, he said.

There is an active search going on. He should be considered armed and dangerous and is a threat to anyone who might approach him, the policeman said.

Watertown Police Chief Ed Deveau called on Watertown to be calm at this time. He said the area affected was east Watertown: "Dexter, Laurel Street, School Street area". That's where people should remain in their homes, Deveau said.

The first policeman said there were no planned controlled blasts at this time. Teams were recovering devices from the suspects' vehicles and making them safe. "Some went off. Some, we believe did not."

He said he would not identify either of the suspects.

Boston police commissioner Edward Davies said of Suspect 2: