A taxi driver who witnessed the aftermath of the incident in Edmonton, north London, told ITV News he saw police force their way into a house on Nightingale Road.
"They broke the door down and brought a man out," he said.
"There were dozens of police cars and a helicopter."
He added: "It's really worrying. It's very scary - we're always on the lookout."
More top news
Donald Trump says the "special relationship" between the UK and the US would improve if he becomes US President.
Former BBC Apprentice contestant Stuart Baggs has died, aged 27, a spokesman has said.
Tonight: a woman awaiting a double hand transplant, a dream realised through strangers, and a family who quit their life to travel Britain.