Two more "significant arrests" were made on Friday for the Westminster terror attack as investigators continue to question eight suspects arrested earlier during raids across Britain.

The toll, including the attacker, rose to five after 75-year-old Leslie Rhodes died on Thursday evening following the carnage claimed by the Islamic State (IS) terror group, the Metropolitan Police said. 

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Police officials identified the attacker as 52-year-old British Muslim convert Khalid Masood, who was born as Adrian Elms in Kent and was shot dead by the police during the Westminster attack, reported the Telegraph.

Masood, a father of three, claimed he was an English teacher. Hours before carrying out the attack on Wednesday, he told the staff of Preston Park Hotel in Brighton, where he was staying, that "London isn't like what it used to be".

Two people remain in critical condition after Masood ploughed a car down Westminster Bridge and stormed the Parliament's estate armed with two blades, fatally knifing police official Keith Palmer.

The victims include Britons, French children, Romanians, South Koreans, Greeks, and people from Germany, Poland, Ireland, China, Italy and the US. Three police officers were injured, two of them seriously.

The Scotland Yard's top anti-terror officer Mark Rowley said at least 50 people were injured with 31 requiring hospital treatment, The Telegraph said. 

Rowley said the police investigation focuses on understanding Masood's motivation, preparation and associates. 

More candlelit vigils for the victims were scheduled on Friday in Birmingham and London. 

According to the daily, Masood was investigated by intelligence agency MI5 for "violent extremism" but was ruled out as a threat by security services before being "re-radicalised".

He is thought to have been radicalised in prison. In 2000, he was jailed for slashing a man across the face in an argument which had "racial overtones". 

He was again charged in 2003 with grievous bodily harm, after stabbing a 22-year-old man in the nose in Eastbourne.

Police and the security services now face serious questions about what they previously knew about the British Muslim convert, who had a string of criminal convictions for assault and had spent time in jail. 

Anticipating criticism, Prime Minister Theresa May announced in the House of Commons that Masood was known to MI5 but insisted he was a "peripheral figure" in an investigation "some years ago" and was "not part of the current intelligence picture".

Meanwhile, members of London's Muslim community have created an online platform to raise funds for the victims of the Westminster attack. The project, called "Muslims Unite for London", has so far received almost 18,000 pounds ($22,000).