A Port of London Authority (PLA) spokesperson said: “We are aware of an incident involving a police vessel at Westminster Bridge.
“We are supporting the response and providing assistance where needed, working closely with the Metropolitan Police, HM Coastguard and partner agencies to manage the situation.”
A police helicopter and an air ambulance were seen circling above.
Some entrances to Westminster Underground station have been shut, Transport for London (TfL) has confirmed.
Uber Boat, which operate the Thames Clippers, said some of its services may be subject to disruption or delay as a result of the incident.
Mother and son Cecilia Lusevic, 56, and Mateo Bilis-Lusevic, 12, said they heard a loud bang when the police boat crashed into Westminster Bridge and then watched as the officers were rescued from the Thames.
The visitors from Chile were on a river tour boat on the north side of the central London bridge when they saw the incident. They were evacuated from their vessel shortly after.
Mateo 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.”
He said the officers were in the water for about 10 minutes and a life ring was thrown for one of them.
A black rigid inflatable boat arrived to help them out of the water – the same boat that witnesses said they saw paramedics treating the casualties aboard.
Mateo said emergency services then also arrived.
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