British police said on Thursday a man had been arrested and armed officers were seen on Whitehall, the street home to numerous government ministries in central London, with a BBC reporter saying two knives had been seen.
Police said no one had been injured in the incident close to Prime Minister Theresa May's official Downing Street residence in Westminster but gave no further details. May was not at Downing Street at the time of the incident, a spokesman said.
A BBC Home Affairs correspondent said on Twitter that two knives had been seen on the ground including a large "breadknife" and added a photograph of an officer wearing a blue forensic suit photographing what appeared to be a knife on the sidewalk next to a rucksack.
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.