Daily Mail journalist Quentin Letts said: "This man had something in his hand, it looked like a stick of some sort and he was challenged by a couple of policemen in yellow jackets and one of the yellow jacketed policemen fell down.

"And we could see the man in black moving his arm in a way that suggested he was either stabbing or striking the yellow jacketed policeman and one of the policemen then ran to get help which was very quick to come."