Police made the arrests overnight in the West Midlands and the North West of England, Mark Rowley, the Met Police Assistant Commisioner, said during a briefing this morning.
The number of arrests has now risen to ten, with nine still in custody after a woman from Birmingham was released on bail.
He revealed two of the victims of Khalid Masood's attack remain in critical condition in hospital, with one in a life-threatening condition.
Masood's birth name was Adrian Russell Ajao, Scotland Yard added.
Yesterday terror group Islamic State (ISIS) claimed Masood was a "soldier of the caliphate".
Answering a question about how he had been radicalised, Mr Rowley, said: "Clearly that’s a main line of our investigation. What led him to be radicalised? Was it through influences in our community, influences from overseas or influences from online propaganda?
"Our investigations, our arrests will help in that but the public appeal will make a big difference in that, if people come forward with more information.
Mark Rowley said two more arrests were made
"It would be wrong for me to say any more than what the Prime Minister said in Parliament yesterday.
"But, we’re looking at his history. Several years ago he had been a peripheral figure, he’s never been part of the mainstream intelligence picture about terrorism."
It is believed Masood may have been radicalised in jail or in Saudi Arabia where he spent four years and converted to Islam.
After he returned to the UK he taught English at a centre in Luton, Bedfordshire, and in 2012 set up his own business in the West Midlands, it is claimed.
The fourth victim was named as 75-year-old Leslie Rhodes, of Streatham, south London.
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He died in hospital yesterday after his life support was pulled.
Mr Rowley said two police officers injured on Westminster Bridge as they walked back from a commendation ceremony remain in hospital with "significant injuries".
Giving further information on how the investigation was going, Mr Rowley said searches at five addresses are ongoing and 16 have finished.
He added detectives have seized 2,700 items from the searches, including "massive amounts of computer data".
Officers have spoken to about 3,500 witnesses, including 1,000 who were on Westminster Bridge and about 2,500 who were in Parliament.
Mr Rowley added any information from the public about Masood would be extremely helpful.
He added: "We remain keen to hear from anyone who knew Khalid Masood well, understands who his associates were and can provide us with information about places he has recently visited.
"There might be people out there who did have concerns about Masood but did not feel comfortable for whatever reason in passing those concerns to us."