BBC political correspondent Iain Watson shares an image of children leaving the area.

Mark Rowley, assistant commissioner of the Metropolitan Police, said the police were  "satisfied at this stage it looks like there was only one attacker".

The man believed to be the attacker is dead, he said.

Four people have died and at least 20 people were injured in the attack.

"Our strength as the city depends on our ability to stand together at such a terrible time," he said.

Four people died in the attack, police have confirmed.

The Muslim Council of Britain has issued a statement on the attack, which has not yet been claimed by any group.

Image source, PA

Hundreds of people were evacuated from Parliament to Westminster Abbey, the Press Association reports. 

Addressing the crowd, which included ministers, MPs, peers and tourists, from the pulpit a Met Police superintendent said it would take around 90 minutes to process everyone. 

Police said people would be assessed to see if they have any information. A prayer was then said for the victims of the attack. 

Labour MP Mary Creagh arrived at the scene shortly after the incident.

She told the BBC: "When we were on our way to vote there were armed police officers standing at the escalator.

"As I got off the escalator I saw people running towards me. Penny Mordaunt [another MP] ran up and said 'shots have been fired, we need to get out now'.

"We had no choice but to go out of the revolving doors one at a time - I shouted to the security guard 'you've got to open the doors' and he then opened them. 

"I found myself in Westminster Tube station, I thought the one thing we don't want is tourists wandering in with their children into this situation.

"I went into the control room and told the controller 'there's been what we think is a terror attack on the Palace of Westminster and I said 'you need to shut the station now'."

Image source, AP

A man suspected of fatally stabbing a police officer at the Palace of Westminster is treated by emergency workers after he was shot by armed police.

A knife can be seen lying on the cobbles nearby.

Alex Homer
BBC Local Live

The head teacher of Holy Family Catholic Primary School in Small Heath, Birmingham, Peter Foley, has reiterated the pupils who had been visiting Parliament are safe.

The school Twitter account published a photo of the children there shortly before the terrorist incident on Westminster Bridge unfolded.

He said: “All the children on the school trip are safe and their parents have been informed of the situation. The deputy head teacher is with the children and is providing me with regular updates.”    

The US Department of Homeland Security has issued a statement condemning Wednesday's attack. 

Image source, PA

Holyrood's Presiding Officer the Rt Hon Ken Macintosh MSP said:

"Members of all parties represented in this Parliament will want to join me in expressing our heartfelt sympathy for all those affected by the tragic events at Westminster. 

"As the true seriousness of the incident emerged, and as an expression of our support for our sister Parliament, parliamentary business at Holyrood was suspended this afternoon for the remainder of the day. 

"No matter how significant our debate might be, to have continued further would not have been appropriate. 

"It was increasingly apparent from the chair as reports emerged that the thoughts and concerns of members lay elsewhere. 

"While there is no intelligence to suggest a specific threat to Scotland, Edinburgh or Holyrood, security at the Scottish Parliament has been heightened with immediate effect as a precautionary measure. 

"Parliamentary business will proceed tomorrow and I will be convening a meeting of Business Managers first thing in the morning to consider the rescheduling of this afternoon’s debate. I am grateful to all of our security staff, and the parliament’s police unit for their assistance today."

Tom Edwards
Transport Correspondent, BBC London

I'm on Waterloo Bridge where not much traffic is moving because of the road closures in place.

No boats are moving on this part of the River Thames.

You can also see people on the London Eye but it is not moving.

BBC Newsnight presenter Evan Davis was at the scene of the incident. Here's one of the photos he took.

The French newspaper  Le Telegramme, external is now reporting that three pupils from the Concarneau high school in Brittany are thought to be among those wounded in the incident.  

It says the trio, aged between 15 and 16, were walking down Westminster Bridge when a car knocked them down.   

One of them ended up on the car bonnet, according to the other young people, the newspaper reports.