Thousands of mourners have filled Trafalgar Square for a vigil for the victims killed in Wednesday's car and knife attack in the heart of London.
MPs and members of the Metropolitan Police were among those who held lit candles in honour of the victims on Thursday.
Police raised the death toll from the attack to five after the death of a 75-year-old. This included the attacker.
"Those evil and twisted individuals who tried to destroy our shared way of life will never succeed and we condemn them," Mayor Sadiq Khan said in an address to the crowd.
"London is a great city full of amazing people from all backgrounds and when Londoners face adversity we always pull together. We stand up for our values and we show the world we are the greatest city in the world," he said.
Police have identified the assailant in the attack as Khalid Masood, 52, who was born in Kent, southeast England.
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Masood drove into pedestrians who were on the Westminster Bridge before being shot dead by police after fatally stabbing Police Constable Keith Palmer.
- Kurt W Cochran from Utah, US was on the last day of a European trip celebrating his 25th wedding anniversary when he was killed on Westminster Bridge. His wife was seriously injured and remains in hospital.
- Aysha Frade, a British citizen whose mother is Spanish, was also killed on the bridge.
- Police Officer Keith Palmer, 48, was on duty protecting parliament when he was stabbed to death.
- A 75-year-old man who had been receiving medical treatment following the attack had life support withdrawn on Thursday evening.
- Attacker Khalid Masood, 52, was shot dead by police.
Twenty-nine people injured in the attack are still being treated at the hospital. Seven are in a critical condition.
Police said Masood had a string of criminal convictions and had been most recently living in central England.
"Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack," a police statement said.
"However, he was known to police and has a range of previous convictions for assaults, including GBH [grievous bodily harm], possession of offensive weapons and public order offences."
He had not been convicted previously for any terrorism offences, it said.
In a statement to the House of Commons on Thursday, Prime Minister Theresa May said Masood was once investigated by intelligence officers over concerns of "violent extremism".
"He was a peripheral figure," she said. "The case is historic, he was not part of the current intelligence picture."
The Islamic State of Iraq and the Levant (ISIL) group claimed responsibility on Thursday for the attack. It said on its Amaq website the attacker "carried out the operation in response to calls for targeting citizens of the coalition" of countries fighting ISIL in Syria and Iraq.
It was not possible for Al Jazeera to independently confirm the claim, which did not offer any details of the attack or name Masood, casting doubt on whether there is any direct link between ISIL and the London killings.
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Joseph Downing, from the London School of Economics, expressed scepticism over ISIL's claim.
"To me, this is something quite common over the last couple of years, over the terrorist attacks in Europe, that ISIL jumps on the bandwagon in the most horrific way and says 'yeah, this our soldier', when there's actually no link between the person carrying out the attack and any particular group," he told Al Jazeera.
Police said eight people had been arrested after raids on six homes in London, Birmingham and other parts of the country in their investigation into the attack.
Al Jazeera's Barnaby Phillips, reporting from London, said: "The absolute priority of the police at this point in time would be to know what sort of accomplices, if any, the assailant had. What sort of assistance, if any, did the assailant have and whether he belonged to any sort of network."
May said that those wounded in the attack included 12 Britons, three French children, two Romanians, four South Koreans, two Greeks, and one each from Germany, Poland, Ireland, China, Italy and the United States.
Three police officers were also wounded.
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.
Source: Al Jazeera and news agencies