The gates of Buckingham Palace were closed over two hours after a knife wielding man stabbed a police officer in Parliament and was shot, as the search for a second assailant continued.
The doors to Britains Parliament meanwhile, which had been on lockdown all afternoon, were opened and the members there were being told to leave the premises.
Four people have died and at least 20 were injured in the terrorist attack near the British parliament, including a police officer and one attacker, police said.
Police are still searching for one of the two people believed to have launched Wednesday's attack outside the British parliament building, assistant editor of the Daily Telegraph newspaper Christopher Hope tweeted, citing senior government sources.
A BBC journalist said two assailants were believed to have been in a vehicle involved in the attack. "BBC understands from multiple sources two assailants in vehicle on Westminster Bridge," BBC correspondent Dominic Casciani said on Twitter.
The main assailant shot by police was captured alive and taken to St. Thomas' Hospital for medical treatment. He was seen there on a stretcher surrounded by police, but officials reported that he died.
The BBC reported earlier that Prime Minister Theresa May was safe, and had been seen being ushered into a silver Jaguar as what sounded like gunfire rang out at Parliament during the incident. One MP inside parliament, said she heard four to five gunshots.
May later announced that she was getting ready to chair a meeting of the emergency security committee.
Both the Metropolitan Police and the Mayors office were cautious about giving out information about the event, but continued to define it as a terrorist attack. Londons police commander said that a full counterterrorism investigation, was underway.
It remained unclear whether a second incident soon after, involving a large vehicle" that eyewitnesses described as plowing through the crowds and hitting at least five people - was related. That incident took place on Westminster Bridge, right outside Parliament. At least two people were reported killed and several others were said to have suffered catastrophic injuries, in that bridge attack.
Scotland Yard put out a statement confirming that it was called to a firearms incident on Westminster Bridge and verified reports of injuries.
Transport for London added, in a tweet, that the Westminster underground station was closed at the police's request, and buses diverted. The Metropolitan Police was asking drivers and pedestrians to stay clear of the area as the situation continued — and to alert them to any information that could be helpful. Londoners were also being asked to refrain from posting photos and video on social media of any of the events and especially of those injured.
The iconic London Eye Ferris wheel within view of Parliament was suspended for close to an hour, according to CNN - with those riding it left suspended over the Thames.
Want to enjoy 'Zen' reading - with no ads and just the article? Subscribe todaySubscribe now