UPDATED 12.54pm Police in London say a 75-year-old man has died from injuries he suffered during the Westminster terror attack.

That takes the death toll to five, including the assailant who's been named as 52-year-old Khalid Masood.

40 people were injured.

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Police say the 52-year-old Briton had a range of previous convictions for assaults and possession of weapons, he was earlier on Thursday claimed as a "soldier" of Islamic State by the terrorist group.

Masood on Wednesday ploughed a car into pedestrians on London's Westminster Bridge before fatally stabbing a police officer inside the gates of parliament, before the attacker was shot dead.

Four people were killed, including the assailant, in the heart of Britain's seat of power, in what British Prime Minister Theresa May condemned as a "sick and depraved terrorist attack".

May also confirmed Masood was known to intelligence authorities.

"What I can confirm is that the man was British-born and that some years ago he was once investigated by MI5 in relation to concerns about violent extremism," May told MPs in her first House of Commons speech after the attack.

"He was a peripheral figure," she added. "The case is historic, he was not part of the current intelligence picture."

She said there had been no prior intelligence of his intent nor of the plot.

Metropolitan Police counter-terrorism chief Mark Rowley on Thursday revised the death toll down to four, including the attacker, a police officer guarding parliament and two civilians.

He said that 29 people were in hospital and seven were in critical condition.

UK police have raided six addresses, including some in Birmingham, and arrested seven people in connection with Wednesday's attack.

An Australian permanent resident named by News Corp as Trish Neis-Beer, a German national who lives and works in the Adelaide Hills, was among those injured in the attack.

Australian Attorney-General George Brandis told the Senate on Thursday that she was receiving treatment in hospital and consular staff were providing assistance.

An American man from Utah, named as Kurt Cochran was among the dead, his wife Melissa among the injured, the duo had been celebrating their 25th wedding anniversary.

Three police officers, several French teenagers on a school trip, two Romanian tourists, and five South Koreans were among the injured casualties.
Rowley identified the police officer who died as Keith Palmer, 48, a husband and father.

Met Police say they'll be retiring Mr Palmer's shoulder number. 4157U will be taken out as a mark of respect, and never re-issued to any other officer.

Islamic extremism was suspected in the attack, Rowley said, adding that authorities believe they know the assailant's identity but would not reveal it while the investigation was ongoing.

The threat level for international terrorism in the UK was already listed at severe, meaning an attack was "highly likely".

Wednesday was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people last year, and the latest events echoed recent vehicle attacks in Berlin and Nice, France.

Conservative MP Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed first aid on the wounded police officer, who later died. Metres away lay the assailant.

"I tried to stem the flow of blood and give mouth to mouth while waiting for the medics to arrive but I think he had lost too much blood," Ellwood said. "He had multiple wounds, under the arm and in the back."

The attack began early on Wednesday afternoon (Thursday AEDT) as Masood, driving a grey car slammed into pedestrians on the bridge linking parliament to the south bank of the River Thames.

Former Polish foreign minister Radek Sikorski was in a car crossing the bridge when he heard "something like a car hitting metal sheet" and then saw people lying on the pavement.

"I saw one person who gave no signs of life. One man was bleeding from his head. I saw five people who were at least seriously injured," Sikorski told Poland's TVN24.

Police said one injured woman was pulled from the river.

The car crashed into railings on the north side of the bridge, less than 200 metres from the entrance to parliament. As people scattered in panic, witnesses saw a man holding a knife run towards the building.

"The whole crowd just surged around the corner by the gates just opposite Big Ben," said witness Rick Longley.

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can't believe what I just saw."

Daily Mail journalist Quentin Letts said a man in black attacked the police officer before being shot two or three times as he tried to storm into the building.

"As this attacker was running towards the entrance two plain-clothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell," Letts told the BBC.

 

WHAT WE KNOW ABOUT THE VICTIMS OF THE LONDON TERRORIST ATTACK

KEITH PALMER
Palmer, 48, had been a member of Britain's parliamentary and diplomatic protection forces for 15 years and a soldier in the Royal Artillery before that.
He was on duty protecting Parliament when he was stabbed to death on Wednesday.

Honouring Palmer, Prime Minister Theresa May said he was "a husband, a father ... he was every inch a hero.

"His actions will never be forgotten."

Conservative MP James Cleverly tweeted he was "heartbroken", having known Palmer for 25 years.

He said they served in the military together.

The Charlton Athletic Football Club said Palmer was a long-time fan and "a familiar face" at its London stadium, The Valley. As a tribute, the club placed one of its red-and-white scarves on his seat in the East Stand where he sat "for many years".

AYSHA FRADE

Frade was a British national whose mother is Spanish, she was one of two people killed on the bridge.

In the north-western Spanish town of Betanzos, where her mother was born and her two sisters run an English-language school, the mayor said: "The whole town is shocked."

Although Frade, 43, was born and lived in London, she spent weeks every summer in Betanzos visiting relatives, said Ramon Garcia Vazquez, mayor of the town of 13,000 people in Spain's Galicia region.

Betanzos town hall held a minute's silence for the family at noon Thursday and the mayor announced three days of mourning, cancelling all official activities in the town.

Frade worked as an administrator at the DLD College - a school in Westminster a stone's throw from parliament.

"She was highly regarded and loved by our students and by her colleagues. She will be deeply missed by all of us," said principal Rachel Borland.

KURT COCHRAN
Cochran from Utah was on the last day of a European trip celebrating his 25th wedding anniversary when he was killed on Westminster Bridge.

His wife, Melissa, was seriously injured and remains in hospital. She suffered a broken leg, a broken rib and cuts and bruises.

They were visiting her parents, who are serving as Mormon missionaries in the British capital, a church spokesman said.

The couple ran a recording studio in the basement of their home just outside Salt Lake City.

Pictures on Kurt Cochran's Facebook page show the couple enjoying their trip through Europe prior to the attack.

VICTIM NO.4

A fourth victim of Wednesday's attack on Westminster Bridge died on Thursday, London police said, without releasing his name or nationality.

The 75-year-old man had been receiving medical treatment in the hospital following the attack and life support was withdrawn on Thursday evening.