By Michael Holden and Kate Holton
LONDON (Reuters) - 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.
Meanwhile police are continuing to question the man who was detained in a dramatic stop-and-search operation on Parliament Street, a stone's throw from the parliament building, May's office and government departments.
No one was injured in the incident and photographs showed three knives on the ground where he was arrested.
Sources familiar with the investigation told Reuters the man was Khalid Omar Ali from London.
The sources, speaking on condition of anonymity, said Ali had been on board the Mavi Marmara, part of a flotilla which was challenging an Israeli naval blockade of the Gaza Strip when it was intercepted by the Israeli Defence Forces in May 2010.
Basu said the suspect was being questioned on suspicion of terrorism offences and two properties were being searched in connection with the investigation.
A Western security source told Reuters the man was on the radar of counter-terrorism officers and the domestic intelligence agency MI5 before his arrest.
Since August 2014, Britain has been on its second-highest alert level of "severe", meaning an attack by militants is considered highly likely. Senior officers said the security services had thwarted 13 terrorism attempts in the last four years shortly before the deadly Westminster Bridge incident.
"Activity continues around the clock to identify and stop these threats and we are making arrests on a near daily basis," Basu said.
(Additional reporting by James Davey in London and Mark Hosenball in Washington, editing by Guy Faulconbridge/Jeremy Gaunt)