London: British police say a man carrying knives has been arrested in Whitehall, the street home to numerous government ministries in central London.

The 27-year-old was arrested on suspicion of preparing and instigating acts of terrorism, police said.

He was stopped near Parliament Square – the site of last month's terror attack on Parliament House and nearby Westminster Bridge which killed five people – after a stop and search operation by counter-terrorism officers, London's Metropolitan Police said in a statement.

Police said no one had been injured in the incident, which occurred close to Prime Minister Theresa May's official Downing Street residence in Westminster.

Mrs May was not at Downing Street at the time of the incident, which occurred at 2.20pm (11.50pm AEST).

A BBC Home Affairs correspondent said on Twitter that two knives had been seen on the ground including a large "breadknife". He tweeted a photo of an officer wearing a blue forensic suit photographing what appeared to be a knife on the footpath next to a backpack.

The suspect is in custody in a south London police station, and police said that although the investigation is ongoing, there is no immediate known threat.

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

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 the Houses of Parliament.

Tory MP James Cleverly was close friends with Keith Palmer – the police officer killed in last month's attack – and paid an emotional tribute to him in Parliament.

He praised the bravery of his late friend's colleagues in a series of tweets after Thursday's events.

Mrs May also thanked police for their handling of the incident.

"I think it shows that our police and our intelligence and security services are on the alert, as they always are, looking to keep us safe and secure," she told Sky News.

Fairfax Media, Reuters