Daily Mail journalist Quentin Letts whose office overlooks the scene told the BBC of the moment police shot the man.
"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.
"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 and as this attacker was running towards the entrance used by MPs to get into the House of Commons.
"As he was running he ran, about I'd say 15 yard perhaps, two plain clothes guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell."