Four people were killed and at least 20 wounded in London on Wednesday after a car ploughed into pedestrians and an attacker went on a stabbing spree with a knife before being shot dead close to the British parliament.

London Metropolitan Deputy Police Commissioner Mark Rowley told reporters a "full counterterrorism investigation" into the attack was under way.

He said the violence started when a car was driven over Westminster Bridge, hitting and injuring a number of civilians and three police officers on their way back from a commendation ceremony.

The car then crashed close to parliament where at least one man armed with a knife continued the attack and tried to enter the building.

Many people were lying on the ground - some bleeding heavily and apparently unconscious - on Westminster Bridge near Britain's parliament.

Some of the wounds suffered by the victims were described as "catastrophic". One woman was pulled out alive from the River Thames with serious injuries by port authorities.

Prime Minister Theresa May condemned the attack as "sick and depraved".

"The location of this attack was no accident," she said in a statement outside her Downing Street office late in the evening.

"The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech."

Witnesses initially reported multiple attacks near parliament.

"We saw a black vehicle at full speed and it ran down a number of people. I could see people flying all around," tourist Babi Nagy told Al Jazeera. "Immediately it came to mind this was a terrorist attack."

Polish politician and journalist Radoslaw Sikorski posted a video on Twitter of the aftermath on the bridge, showing several injured people lying on the ground.

A car on Westminster Bridge has just mowed down at least 5 people.pic.twitter.com/tdCR9I0NgJ — Radosław Sikorski (@sikorskiradek) March 22, 2017

Another witness said he saw victims scattered along the street.

"As I was walking up the steps, there was a man who had fallen and medics were taking care of him. There was a lady who was also stabbed or shot. There was a lot of blood," Martin Pearce, visibly shaken, told Al Jazeera at the scene.

Security analyst Afzal Ashraf said extra vigilance by the public and authorities was now necessary.

"One of the features of these forms of attacks is a follow-on attack. They like to have publicity. The potential threat is huge," Ashraf said.

The last major attack to hit London was in July 2005, when a coordinated series of bomb blasts targeted its public transportation system during rush hour. The bombings killed 52 people and wounded more than 700 others.

London Mayor Sadiq Khan said on Twitter that an urgent investigation had been launched. "My thoughts are with those affected and their families," he said.

International messages of condolence were offered to the United Kingdom.

Chancellor Angela Merkel said Germany stood by Britain following the deadly attack.

"Although the background to these acts are not yet clear, I reaffirm that Germany and its citizens stand firmly and resolutely alongside Britons in the struggle against all forms of terrorism," Merkel said in a statement.

US President Donald Trump offered May the full cooperation and support of the United States in responding to the attack, the White House said in a statement.