"Officers are on the scene." An AFP photographer saw firearms officers surrounding the man and pinning him to the ground at a pedestrian island.
David Wisniowski, who was working on a building site right next to the incident, told AFP he could see "three knives on the floor, one big one and two small".
Gabrielle Hennessy, 20, a US tourist, described seeing police rush at a man.
"We were walking along Parliament Square and we saw police rush at him. And then he was on the ground surrounded by police. They stood him up and that's when I saw the knife on the ground," she told AFP.