Armed British police officers have arrested a man carrying knives near Prime Minister Theresa May's office in Westminster on suspicion of preparing an act of terrorism.

LISTEN ABOVE: UK correspondent Ben Bland speaks to Nadine Higgins 

The 27-year-old man was detained on Thursday afternoon by counter-terrorism officers on Parliament Street, a stone's throw from the UK parliament building and May's Downing Street office, during a stop and search as part of an ongoing operation, London police said.

The London man detained was known to counter-terrorism officers and Britain's domestic intelligence agency MI5 before his arrest, a Western security source told Reuters.

No one was injured in the incident, and May's spokesman said the prime minister was not at Downing Street at the time.

The incident comes just over a month after a British-born convert to Islam ploughed a car into pedestrians on nearby Westminster Bridge, killing four people, before stabbing to death a police officer in the grounds of parliament.

"Detectives from the Counter Terrorism Command are continuing their investigation, and as a result of this arrest there is no immediate known threat," police said in a statement.

"The man ... was arrested on suspicion of possession of an offensive weapon and on suspicion of the commission, preparation and instigation of acts of terrorism. Knives have been recovered from him."

The BBC said the man was being followed and tracked by police as part of a counter-terrorism operation when the arrest took place.

Pictures on social media showed a large group of heavily armed officers surrounding the suspect.

A Reuters reporter and photographer saw police detaining a dark-skinned, bearded man dressed in a hooded, black coat. His right hand appeared to have a bandage wrapped around it.

Reuters pictures showed police officers standing by three knives lying on the ground. Police said the man was being held at a south London police station.