Update: 21:26

A fifth person has died following the Westminster attack in London, as crowds of people have gathered in Trafalgar Square in London to remember the victims.

The vigil was organised by the city’s mayor Sadiq Khan following yesterday's attack that killed three people and injured 29 outside the Houses of Parliament.

Mr Khan invited both residents and tourists to join together "to show the world that we are more committed than ever to the values that we hold dear; that we remain united and open."

Three candles were lit in memory of the victims - London policeman Keith Palmer, American tourist Kurt Cochran and London mother of two, Aysha Frade.

The family of PC Keith Palmer, who was stabbed to death, said he was proud to be a police officer - while British Prime Minister Theresa May has called him "every inch a hero."

Witness Tony Davis jumped a fence yesterday, to give the police officer first aid:

The attacker – who was shot dead by police – has been named as British-born Khalid Masood.

Of the 29 people taken to hospital, seven are said to be in a critical condition.

Crowds at the vigil, 23-03-2017. Image: Kieran Cuddihy

The Deputy Commissioner of the London Metropolitan Police Craig Mackey, British Home Secretary Amber Rudd and London Mayor Khan spoke at the event, before a minutes silence.

Addressing the crowds, Mr Khan described the incident as an “attack on our shared values” and told the assembled public, “when Londoners face adversity we always pull together.”

"We come together as Londoners tonight to remember those who have lost their lives and all those affected by the horrific attack yesterday," he said

"But also to send a clear message - Londoners will never be cowed by terrorism.

"We stand up for our values and we show the world we are the greatest city in the world."

"London will never be beaten. We stand as one and united," 23-03-2017. Image: Kieran Cuddihy / Newstalk

British Home Secretary Amber Rudd was applauded when she told the crowds “we will defeat” terrorism.

"We showed that by coming together, by going to work, by getting about our normal business because the terrorists will not defeat us," she said.

"I want us to say thank you to all for the great sacrifice the police and emergency services take."

The vigil was attended by religious leaders from all faiths, who were united in their grief and solidarity with the victims.

Religious leaders arriving at the vigil, 23-03-2017. Image: Kieran Cuddihy / Newstalk

Earlier, Theresa May said the attacker was a "peripheral" figure who had once been investigated by MI5 over concerns about violent extremism.

While he was born in Kent, detectives believe he had recently been living in the West Midlands of the country.

He had a range of previous convictions, however police said he was not the subject of any current investigation and had never been convicted of any terrorism related offences. 

In a statement, the Metropolitan Police said there was "no prior intelligence about his intent to mount a terrorist attack."