The Metropolitan Police say two officers have been taken to hospital after being injured in central London.
The incident, which happened near Leicester Square around 06:00 Moscow time, is not considered to be related to terrorism.
Nothing is known about the condition of the officers.
A taser was used and a man was arrested on suspicion of grievous bodily harm and assaulting an emergency worker, Scotland Yard said. He is also being treated in hospital.
London Mayor Sadiq Khan called the attack "absolutely horrific".
He said: “These brave officers were doing their duty and helping the community at this momentous time for our country.
"My thoughts and prayers are with them, their loved ones and their police colleagues after this shameful attack," Khan said.