US President Donald Trump's son has slammed the Mayor of London for saying terror attacks are "part of living in a big city" as his father called the Parliament attack "big news".

Donald Trump Jnr tweeted a link to a story from the Independent about comments made by London Mayor Sadiq Khan in September.

14 President Donald Trump's son has slammed the Mayor of London for saying ‘terror is part of living in a big city’ Credit: CQ-Roll Call Group

14 Donald Trump Jnr, tweeted a link to a story from the Independent about comments made by London Mayor Sadiq Khan in September

Mr Khan said he believes the threat of terror attacks are "part and parcel of living in a big city".

He added: "That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but it also means exchanging ideas and best practice."

Trump Jnr tweeted a link to the story adding: "You have to be kidding me?!"

Earlier when entering a meeting in the White House's Roosevelt Room Mr Trump said the attack in London was "big news".

His press secretary Sean Spicer tweeted the President "has been briefed on the situation" in London and was monitoring events.

Mr Spicer also added the President had also spoken to Prime Minister Theresa May about the attacks.

The US State Department says it is closely monitoring the incident and urged Americans in London to avoid the area.

Spokesman Mark Toner said: "We stand ready to assist in any way the UK authorities would find helpful."

He added the US Embassy in London is closely following the news and stands ready to help any Americans caught up in the attack.

He said: "Our hearts go out to those affected."

14 Sean Spicer tweeted that the President is keeping a close eye on events Credit: Reuters

14 Paramedics treat a victim of the attack on Westminster Bridge Credit: AP:Associated Press

Armed police shot a suspected attacker outside the Houses of Parliament this afternoon after an officer was stabbed in what police said was a "terrorist" incident.

The building was immediately sealed off and MPs and staff ordered to remain inside.

Witnesses also said a car mowed down a dozen pedestrians on Westminster Bridge before crashing into the gates outside Parliament.

14 The incident comes with Europe on high alert after a series of deadly jihadist attacks

One woman has died, the Press Association reported, quoting a doctor. Other people are being treated for "catastrophic" injuries.

An announcement made in Parliament said there was a suspicious package in a vehicle and the bomb squad had been called which halted a planned evacuation.

David Lidington, the leader of parliament's lower House of Commons who is responsible for arranging government business, told MPs: "What I am able to say to the house is there has been a serious incident.

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Foreign office minister Tobias Ellwood looking like he's sporting fresh blood #London pic.twitter.com/pEX4Zlsjnr — Hugh Dickinson (@Hugh_Dickinson) March 22, 2017

Bournemouth MP @Tobias_Ellwood helped injured policeman in Parliament - gave mouth-to-mouth. His brother was killed in Bali terrorist attack — Peter Henley (@Peter_Henley) March 22, 2017

14 A knife is seen on the floor at the scene of the terrifying incident Credit: PA

"It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.

"There are also reports of further violent incidents in the vicinity of the Palace of Westminster," he said.

He said he could not go into further details until there was confirmation from the police and security authorities.

Prime Minister Theresa May is safe, a statement from her Downing Street office said, and she was seen getting into a car and being driven away from parliament.

14 The vehicle in the attack in Westminster today

Lidington said an air ambulance was at the scene to take away casualties.

TV pictures showed traffic halted on the nearby Westminster Bridge and emergency vehicles swarming around. The busy bridge was completely shut off to traffic.

Armed police swarmed to the area which was quickly sealed off to the public.

Poland's former foreign minister, Radek Sikorski, says he was in a taxi leaving Westminster and was checking his email when he heard something like a car crash, "something like a car hitting metal sheet."

My statement on the incident near Parliament Square this afternoon. Please visit https://t.co/NzFxlDdZ9N for the latest information. pic.twitter.com/T4s43cGzub — Mayor of London (gov.uk/coronavirus) (@MayorofLondon) March 22, 2017

14 A member of the public is taken away for treatment by emergency services medic Credit: Getty Images

"I look up and I see that a person is lying on the pavement. I started my camera and I saw more people lying on the street and on the pavement," Sikorski said on Poland's TVN24.

"People started running up to them. I saw one person who gave no sign of life, another man was bleeding from his head.

"In all, I saw five people who were at least seriously injured," he said.

14 Cops surround a man on the floor at the scene Credit: PA:Press Association

14 An armed officer inside Parliament

14 A car slammed into railings outside Parliament

14 People attending to an injured person at the Westminster Bridge Credit: Reuters

The incident came on the same day that Belgium was marking a year since its most deadly attack, when 32 people were killed in suicide bombings at Brussels Airport and a metro station.

In July 2005, four British suicide bombers inspired by Al-Qaeda attacked London's transport system during rush hour, killing 52 people.

14 A helicopter was pictured outside Parliament Credit: Reuters

14 A number of bystanders in the area were hurt. Some are reportedly fighting for life Credit: Getty Images

Two weeks later, there was an attempt to carry out a second wave of attacks.

A spokesman for the House of Commons told AFP: "We can confirm that the sitting has been suspended in the Commons at the moment. We are aware of a security incident that has taken place."

London's Metropolitan Police said on Twitter: "We were called at approx 2:40 pm to reports of an incident at Westminster Bridge. Being treated as a firearms incident - police on the scene.

WAS IT A 'LONE WOLF ATTACK'? The mowing down of pedestrian on Westminster Bridge and then the stabbing of a police officer guarding the Houses of Parliament could be the work of a lone wolf, a security expert said. Will Geddes, CEO of ICP Group said the two incidents are highly likely to be terrorist related but it was unclear if they are connected. He added lone wolf attackers are always a challenge to prevent. Mr Geddes said: "There have been two incidents and it is not clear if they are connected. "The first is that someone has stabbed a police officer, he was then shot by another police officer. "There are unconfirmed reports that he is dead. "The second incident is that a number of people, around four or five, have been injured on Westminster bridge by a car. "It could be a lone wolf attack. "I would say at the moment it's looking very likely to be a terrorist attack, whether it is a lone wolf or a group attack is not known."

"Officers -- including firearms officers -- are on scene and dealing with the incident."

A staff member in parliament, who did not want to be named, told AFP: "I definitely heard shots. I saw someone in dark clothing go down."

Westminster station on the London Underground train network, which is opposite the Houses of Parliament, was closed over to the incident.