Helen Chandler Wilde told the BBC: "I saw a couple of police officers with a guy interviewing him and then putting him in a car as well as a couple of police forensics officers photographing the scene.

"And I did see the knives on the floor with the brown backpack lying next to it which was in the middle of the intersection in Whitehall.

"I didn't see the guy, you know, being put on the floor by the police, so all I can imagine is that he was stopped there and dropped his things and then was taken to the other side of the road just outside the Treasury to be arrested and questioned which is where I saw him

"I'd say there were a dozen or two dozen [police officers] kind of spread out around Westminster and Parliament Square.

"He had his head low, his hood still up of his black hoodie, trying to hide his face from police, I don't know, but from photographs he appeared to be smirking.

"[People were] remarkably not scared at all, not worried, the road was shut off but only just, still on the other side of the road there were people on the bus stuck in traffic on the way home just like any normal day, there were even a couple of tourists coming up to me thinking the Queen was coming to visit because the road was shut. I had to disappoint them there."