By Costas Pitas and Estelle Shirbon
LONDON, March 23 (Reuters) - Police arrested seven people in the investigation into a lone-wolf attacker who killed three people and injured 40 before being shot dead by police near parliament in London, Britain's most senior counter-terrorism officer said on Thursday.
Mark Rowley said there were four dead including the attacker and 29 people still being treated in hospital, seven of whom were in a critical condition. Police had said on Wednesday that the death toll was five in the worst such attack in Britain since 2005.
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The attacker sped across Westminster Bridge in a car, ramming pedestrians along the way, then ran through the gates of the nearby parliament building and stabbed a policeman before he was shot dead.
Authorities have described the attack as a "marauding terrorist incident" and said they were working on the assumption that it was Islamist-related.
Britons have been left shocked by the fact that the attacker was able to cause such mayhem equipped with nothing more sophisticated than a hired car and a knife.
Police believe they know the identity of the attacker but have not named him.
Rowley said police had searched six addresses in London, Birmingham and other parts of the country in their investigation.
"It is still our belief ... that this attacker acted alone and was inspired by international terrorism. At this stage we have no specific information about further threats to the public," Rowley said.
He said there was a mix of nationalities among the dead but gave no details. The victims were the policeman who was stabbed and two members of the public, a woman in her mid-40s and a man in his mid-50s. The fourth dead was the assailant.
Three French high-school students aged 15 or 16, who were on a school trip to London with fellow students from Brittany, were among the injured.
French Foreign Minister Jean-Marc Ayrault was expected to arrive in London to visit them at hospital, French media reported.
There were also five South Koreans among the injured, South Korea's foreign ministry said in Seoul.
MAY TO MAKE STATEMENT
British Defence Minister Michael Fallon said security arrangements at parliament would be reviewed.
Westminster Bridge remained cordoned off with a strong police presence. The nearby Westminster underground rail station, normally a busy hub in the morning rush hour, was not accessible from the street as it was within the cordon.
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Parliament was due to convene later in what Prime Minister Theresa May said on Wednesday was a sign that the attack would not disrupt British democracy or normal life in the capital. May was expected to make a statement to parliament.
She said on Wednesday the location of the attack was not an accident. She said any assault on British values of liberty, democracy and freedom of speech was doomed to failure and Britons would not be divided by such acts.
But anti-immigration groups were quick to make links between immigration and the attack.
Leave.EU, a group that has campaigned for immigration to be severely restrained as part of Britain's exit from the European Union, accused mainstream politicians of facilitating acts of terror by failing to secure borders.
"We are sick, tired but perhaps even more so we are angry that recent governments across Europe have enabled these attacks through grossly negligible policies that have left us vulnerable," the group said in a statement.
In France, far-right presidential candidate Marine Le Pen also drew a link, saying that events in London highlighted the importance of protecting national borders and stepping up security measures.
(Additional reporting by Kate Holton and William James, writing by Estelle Shirbon; editing by Guy Faulconbridge and Giles Elgood)