LATEST: London police raise arrest total to eight in terrorism investigation after Westminster attack
Acting London head of police to attend government security meeting on Thursday
The acting head of London's police force Craig Mackey said he would attend a meeting of the government's emergency security committee on Thursday after an attacker killed three people near the British parliament.
Mackey is being treated as a significant witness as he was at scene when the incident took place. He was not one of those injured.
British Prime Minister Theresa May will make a statement to parliament at 1030 GMT.
UK defence minister says assumption is that this is related to Islamic terrorism in some form.
UK defence minister Fallon says police are checking whether other people are involved in the London attack.
UPDATE: Britain's top anti-terrorism officer says - we have searched six addresses and made seven arrests.
JUST IN: Britain's top anti-terrorism officer says seven in hospital in critical condition
JUST IN: Britain's top anti-terrorism officer says attacker attacked alone
JUST IN: Britain's top anti-terrorism officer says death toll from London attack now stands at four, 29 treated in hospital
JUST IN: British police make several arrests in Birmingham raid linked to investigation into attack on parliament - Sky News
JUST IN: British police raid house in Birmingham as part of investigation into Wednesday's attack on parliament - Sky News citing unidentified sources
BREAKING: UK police say death toll rises to 5 people after attack close to parliament building in London, attacker was inspired by international terrorism
UK police say will not comment on identity of attacker at this point