Five officers were injured when a police boat travelling “at speed” struck Westminster Bridge on the Thames.
Three police officers were thrown into the water during the collision before they were rescued by people on nearby boats on Friday.
Police were responding to a report of a person in the river near Battersea.
One witness said the officers were in the water for about ten minutes and a life ring was thrown in for one of them.
Witnesses described hearing an “explosion” and a “big bang” when the boat crashed.
In a statement, the Metropolitan Police said: “At around 3.40pm on Friday, July 24, a boat from the Met’s marine policing unit struck Westminster Bridge while responding to a report of a person in the water near Battersea.
“As a result of the collision, three officers were thrown from the boat into the water. They were recovered by those on nearby boats.
“Additional officers responded alongside the London Ambulance Service and London Fire Brigade.
“Five officers were injured and received treatment at the scene before being taken to hospital. One has since been discharged.”
Traffic and pedestrians were cleared from Westminster Bridge before it was taped off by emergency services.
Hundreds of onlookers were seen crowded along Victoria Embankment as emergency services swung into action.
Responders from the RNLI could be seen treating casualties on a boat at Westminster pier.
A large number of police officers, fire crews and ambulances were on the scene, with traffic across the bridge halted.
Several emergency vehicles were also parked on the bridge and along the Embankment, while a police helicopter and an air ambulance circled above.
A fire service boat was later seen towing the police boat down the River Thames.
The black rigid inflatable police boat was tethered to the larger fire service vessel as it made its way along the water.
Police boats regularly patrol along this stretch of the Thames as part of measures to protect Parliament from a terror attack or intruders.
At times their vessels speed across the water. The arches at Westminster Bridge are large enough for sightseeing boats, which are far bigger than the small police vessels, to pass under.
A couple from the United States said they heard “an explosion” when the boat struck the bridge, adding it was travelling at “high speed” and seemed to be in pursuit of something further up the river.
Jim Bagley, who was visiting the capital from Gloucester, Massachusetts, with his wife said: “We were standing in line to get onto the city cruise hop on, hop off tours, and we saw two police boats coming up the river.
“One was an orange boat that went under the bridge and left a great big wave, and then there was a black boat that came up behind it, the police boat.
“It looks to me that he got stuck in the wake of the other boat that had just passed through the bridge and slammed into the wall on the opposite side.
“We didn’t see it happen, but we heard the explosion… the next thing we saw was three people in the water, coming down the river.”
The couple then saw other boats on the river attempt to save those stranded in the water, throwing a cargo net and a life ring into the water.
Mr Bagley said: “There was another boat that came up from the river that had an open transom, and they were able to back into the people in the water and drag them onto the back of the boat.”
He added: “We were wondering why they were under way at high speed to begin with, we don’t know why they were being called upriver.”
His wife, Katherine, said: “They were both in pursuit of something farther down the river, but they never… the second boat never got to it because it crashed into the bridge with a big bang.”
Mother and son Cecilia Lusevic, 56, and Mateo Bilis-Lusevic, 12, said they heard a loud bang when the police boat crashed and then watched as the officers were rescued from the Thames.
The visitors from Chile were on a Thames tour boat on the north side of the central London bridge when they saw the incident. They were evacuated from their vessel shortly after.
The 12-year-old said: “Two boats were riding extremely fast through the water.
“Suddenly, I heard a boom. I was looking towards the boat, and I heard a boom.
“Then three people went flying into the water and one of the people was really hurt.”
A student said the incident sounded like a “car crash”.
Enno, who did not want to give his last name, 19, from Germany – who is in the capital on holiday, said he was walking over the nearby Hungerford Bridge when he heard a “crashing sound”.
He added: “A black boat had crashed into the side of the bridge by the pier. I think around five police officers were in the water, at the same time.
“They looked hurt, but not that much.”
He said that emergency services then rushed to the scene and urged people to “stay away”.
He added: “It sounded like a car crash from around 10 metres away. I had seen the black boat pass under the bridge and once I heard the sound, I saw that it had crashed.”
An Uber Boat service, operated by Thames Clippers, was cancelled and others delayed.
The RB1 3.26pm Battersea Power Station to Barking Riverside service was cancelled, the company said.
An Uber Boat by Thames Clippers’ spokesperson said: “An incident occurred in the vicinity of Westminster Bridge on Friday afternoon that did not involve Uber Boat by Thames Clippers, however some services may be subject to disruption or delay as a result.
“For further information and updates please refer to the Port of London Authority or the Metropolitan Police.”
London Fire Brigade said its fire boat and three of its fire rescue units with water rescue capabilities were mobilised following the incident.
