Team GB boxer Frazer Clarke says he looked into the eyes of the Westminster attacker and saw a "crazed man" hell-bent on killing as many people as possible.
The super-heavyweight Commonwealth bronze medallist, from Burton upon Trent, was visiting the Houses of Parliament with his British Lionhearts teammates when he came just 20m (66ft) from terrorist Khalid Masood.
Mr Clarke and his friend filmed some of the terrifying events on their phones witnessing the stabbing of PC Keith Palmer.
The boxer said the day had started a special one, and he was enjoying having fun and messing about with his fellow Lionhearts when he went into the courtyard and "all hell broke loose".
He said: "(Masood) didn't intend to hurt or wound anyone - he was trying to kill the officer. That was the only word for it - it was just savage.
"Among all the madness there was one calm figure, and that was the armed police officer. They are trained for this situation, but I don't think you can replicate that sort of situation anywhere, anyhow."
Mr Clarke said the armed officer walked through the "mad crowd" towards Masood, who was walking forwards with a knife in both hands, and shot the attacker twice in the chest.
The athlete says he is sure other people would have died if the armed officer had not taken action.
Mr Clarke was held for about seven hours along with hundreds of other visitors and said the period was terrifying, as police had said an unidentified package was in Masood's car.
"I kept thinking about another attack or a bomb going off," he said. "We were there for our safety but being in that big crowd just made me feel even more of a target."
Once released, the team had to decide whether or not to box the next day. Despite their ordeal, they all fought and won against the visiting Italian team.
Mr Clarke won after being knocked down in the second round and says he's just glad the whole event is over.
:: Police have made a 12th arrest over the Westminster attack after a 30-year-old man was held in Birmingham.
A 58-year-old man remains in custody and a 32-year-old woman was freed on bail.
Nine of the detainees were released with no further action.