A police boat responding to a report of a person in the River Thames crashed into Westminster Bridge in central London on Friday, launching officers into the water and injuring five of them, the Metropolitan Police said.

Paramedics treated the wounded on a black inflatable boat as a police helicopter hovered overhead near the clock tower that houses the bell known as Big Ben.

Four officers remained in the hospital Friday night with serious injuries but were expected to survive, police said.

Witnesses said they saw two boats speeding up the river and heard a loud bang when one struck the bridge.

Jim Bagley, who was visiting from the U.S., said the first boat kicked up a large wake as it went under the bridge and it appeared to push the second boat into the abutment.

"We didn't see it happen, but we heard the explosion," Bagley said. "The next thing we saw was three people in the water, coming down the river."

People on other boats helped pull the officers to safety. Police said no civilians were hurt.

A large crowd gathered to watch as ambulances and fire crews responded but were eventually cleared from the bridge, which was closed to traffic.