UK authorities also lowered the death toll from Wednesday's attack to three, excluding the attacker.
British police carried out raids on six properties and arrested eight people in London and Birmingham on Thursday a day after an attacker killed three people before being fatally shot by police within parliament's grounds.
The attacker acted alone and was inspired by international terrorism, the country's anti-terrorism officer Mark Rowley told reporters.
Rowley lowered the death toll to three, excluding the attacker, from four that was previously announced.
He said that 29 people required hospitalisation, seven of whom are in critical condition.
Three French high-school students, who were on a school trip to London, two Romanians and five South Korean tourists were among the injured.
Rowley refused to provide any information about the victims killed or the attacker.
He also said that more police officers will be on the street reassuring the public but there is no immediate threat of a new attack.
TRT World's Jon Brain has the latest on the investigations.
British police also raided a house in Birmingham on Thursday as part of the investigation.
TRT World's Abubakr al Shamahi is in Birmingham with more.
British Prime Minister Theresa May condemned the attack saying that "the location of this attack was no accident."
Londoners and visitors "will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart," May said.
TRT World's Sarah Morice explains what happened from London.
Britain's parliament reopened on Thursday with a minute's silence in a gesture of defiance against terrorism.
Sombre-looking lawmakers in a packed House of Commons chamber bowed their heads, and police officers also marked the silence standing outside the headquarters of London's Metropolitan Police nearby.
Parliament Square, Westminster Bridge and several surrounding streets remain cordoned off by police; and scores of unarmed officers in bright yellow jackets were staffing the perimeter tape, guiding confused civil servants trying to get to work.
In Parliament's New Palace Yard, a blue police tent was erected over the spot where the stabbing and shooting occurred, and two forensic officers worked at a trestle table nearby.
The attack on London comes exactly a year since the bombings in Brussels and the tactics used are similar to those deployed in Nice and Berlin last year.