Police patrol across Westminster Bridge toward the Houses of Parliament in central London on March 23, 2017 after the bridge reopened to traffic following its closure during the March 22 terror attack.

British counter-terrorism police said on Friday they had thwarted an active plot after a woman was shot during an armed raid on a house in north London in the second major security operation in the British capital in the space of a few hours.

Armed counter-terrorism officers using tear gas stormed a house in the Willesden area of the capital on Thursday evening which had been under surveillance, shooting a woman in her 20s. She is said to be in a serious but stable condition in hospital.

Asked if police had foiled an active plot, Neil Basu, Senior National Coordinator for Counter Terrorism Policing, replied: "Yes".

In a separate incident only hours earlier, a man was arrested carrying at least three knives near Prime Minister Theresa May's office in Westminster during another operation by armed counter-terrorism police.

The 27-year-old, who was being monitored by British intelligence agents and police, remains in custody.

"Due to these arrests that we have made, I believe that we have contained the threats that they posed," said Basu, adding it had been "an extraordinary day in London".

Both incidents occurred just over a month after a British-born convert to Islam ploughed a car into pedestrians on Westminster Bridge, killing four people, before stabbing to death a police officer who was in the grounds of parliament. The man responsible, Khalid Masood, was shot dead at the scene.

Basu said six people had been arrested in the Willesden operation, five at the house and another in Kent, in southeast England. The shot woman has not been arrested and Basu said her condition was being closely monitored and the independent police watchdog had been informed.

"The armed entry was necessary due to the nature of the intelligence we were dealing with and involved armed officers firing CS gas into the address," Basu said. Searches at three houses were continuing, he added.