Scotland Yard said it was "open minded as to whether terrorism was a motive".

A statement said: "At this time it is too early to understand the motivation so we have not declared this a terrorist incident.

"However given the location, the circumstances and recent tragic events, the Metropolitan Police counter terrorism command will be investigating this incident.”

Witnesses heard shouts of "knife! knife! knife!" as the man was tackled at around 11am on Friday.

Bradley Allen, 19, from Barking, east London, said he saw the man with clenched fists moments before he was detained.

Mr Allen said he was walking past Parliament when he saw a man "stood there, who looked quite angry with fists quite clenched".

"He stared at me and my mate and we walked past him. Within seconds there was a shout and police ran out of the front gate shouting at him 'Get on the floor, get on the ground'.