Politicians described chaotic scenes in Westminster after a security alert meant many had to be evacuated from Parliament.

Peer Sheila Noakes wrote: "Shots in Parliament Square. People running like crazy."

Conservative MP Grant Shapps tweeted: "Police response instant. Heard commotion, looked round. Police weapons drawn, 4 shots, police ordered us to hit ground & get back, get back."

Fellow Tory Michael Fabricant described a similar scene, saying: "I was walking briskly to a vote when crowds of MPs rushed towards me saying that shots have been fired and shouts of 'get back'."

Liberal Democrat leader Tim Farron tweeted shortly after news of the incident broke: "Currently with a shell-shocked group of MPs at New Scotland Yard having been evacuated following the attack. Police doing an incredible job."

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Many MPs spoke about being locked inside rooms in the Parliament building. Green Party leader Caroline Lucas tweeted: "Locked in Commons with fellow MPs. Thoughts with innocent people hurt today and thanks as ever to Police & other staff working so hard here."

Labour's David Lammy wrote: "Locked in the Commons Chamber but my heart goes out to the police officers, those hit by the car on the bridge and their families. Stay safe."

Former education secretary Nicky Morgan told the Press Association: "The Division Bell had just gone and MPs were making their way to the House of Commons chamber. I was walking from Portcullis House along the path of Old Palace Yard when suddenly shots rang out.

"It takes a moment to realise that is actually gunfire and at that point people were yelling get down get back.

"They pushed us all back to the new building.

"We have at the back of our mind all the time that something terrible can happen. The first thing is is that actually gunfire? Have you misheard? But the reaction of the officers around made it very clear this is a very serious situation."

Around half an hour after the attack, former cabinet minister Anna Soubry claimed she and the Security Minister, Ben Wallace MP, were ordered into the Parliament lobby "at gunpoint" by police officers.

A Reuters photographer reported seeing a dozen injured at Westminster Bridge. Photo: Reuters/Toby Melville

"Colleagues taken out of the library at gunpoint by counter-terrorist armed police officers," she tweeted.

"Armed officers ordered myself and Security Minister into lobby at gunpoint. Terrifying," she said.

Shadow health secretary Jonathan Ashworth told the Press Association: "I was in my office which is here in Portcullis House on the fifth floor which overlooks Big Ben.

"I actually didn't hear anything. My wife actually rang me to say 'are you all right?' and I said 'what are you on about?'.

"And then she told me and I looked out of the window, and through the window I could see a car which had mounted the pavement, and looked like it had tried to crash into the walls of the old palace bit.

"And then over in Old Palace Yard on the House of Commons side of the gates I could see the police, looked like they were trying to resuscitate somebody on the floor, and I could see a lot of armed police running around and I was in absolute shock.

"Thousands of people work here, it's not just MPs. The police have obviously got to keep everybody safe and I commend them for what they are doing.

"I think everybody is in absolute shock here this afternoon."

Mr Ashworth said his office had been evacuated, with staff taken to Portcullis House, while his wife was in another part of the palace.

He said it is "too early to speculate" on what happened but "whatever happens people can attack this building, and it is symbolic to attack this building, but they will never defeat democracy, they will never defeat the values of liberty, of freedom, of equality, that we stand for".

In a statement Labour leader Jeremy Corbyn said: "Reports suggest the ongoing incident in Westminster this afternoon is extremely serious.

"Our thoughts are with the victims of this horrific attack, (their) families and friends.

"The police and security staff have taken swift action to ensure the safety of the public, MPs and staff, and we are grateful to them."