The British parliament was in lockdown on Wednesday after police shot a man who mowed down several people in his car and then forced his way into the parliament complex and stabbed an officer.
One woman was killed and several others sufferd "catastrophic injuries".
Eyewitnesses saw a vehicle plough into pedestrians on nearby Westminster Bridge, which crosses the River Thames next to the houses of parliament.
St Thomas Hospital, which is located across the River Thames from the Houses of Parliament was cleared of all non-emergency cases to treat the injured.
London’s police force, the Metropolitan police, said they were treating the attack as a terrorist incident "until we know otherwise".
After crashing the vehicle into railings, the driver leapt out and forced his way through the gates into the yard outside the houses of parliament, brandishing a knife.
Police were called to the scene at 2.40pm, after hearing reports of a "firearms incident".
The man with the knife, who has not been identified, is said to have stabbed a policeman before being shot by armed security staff. David Lidington the leader of the House of Commons, told members of parliament who were gathered in the commons chamber that parliament was suspended for the rest of the day and they were all in lockdown.
An emergency helicopter landed in Parliament Square soon after, to attend to the casualties. Parliamentarians and other staff were instructed to stay in their offices, while prime minister Theresa May was taken away to safety.
The London Underground station at Westminster was shut down by police, even as pedestrians were seen running from Westminster Bridge towards the high-security zone of Parliament Square. All approach roads to parliament sqaure were closed off. The identity of the assailant remains unknown.
The threat level for international terrorism in the UK was listed at severe. The city was also on alert for the funeral of Martin McGuinness, former IRA commander, due to take place on Thursday.
Dennis Burns, who was just entering Parliament for a meeting, said he heard a radio message saying an officer had been stabbed. Police and security rushed outside as he was going in.
"When I got inside I was wondering what the hell was going on and I saw dozens of panicked people running down the street," he said. "The first stream was around 30 people and the second stream was 70 people. It looked like they were running for their lives."
On the bridge, witnesses said a vehicle struck several people, and photographs showed a car crashed into railings. The former Polish foreign minister Radoslaw Sikorski posted a video on Twitter showing people lying injured on the bridge. Mr Sikorski, a senior fellow at Harvard Centre for European Studies, said he saw at least five people lying on the ground after being "mown down" by a car.
Political journalist George Eaton said that from the window of Parliament’s Press Gallery, he saw police shoot a man who charged at officers.
"A large crowd was seen fleeing from the man before he entered the parliamentary estate," he said. "After several officers evaded him he was swiftly shot by armed police."
The incident in Westminster occurred the day after the UK said it was imposing a US-style ban on large electronic devices in cabin baggage on incoming flights from Egypt, Jordan, Lebanon, Tunisia, Turkey and Saudi Arabia.
On Tuesday, the day the ban was annunced, transport minister Chris Grayling told parliament Britain was facing "a constantly evolving threat from terrorism and must respond accordingly".

Source: The National