Tommy Robinson said he was acting in “self-defence” when he punched an England football fan to the ground in Portugal.
The incident occurred outside the Nations League semi-final between England and the Netherlands in Guimaraes on Thursday.
Social media footage showed the EDL founder swinging at the man, who falls to the ground.
Robinson said he lashed out at a drunk England fan who had been “verbally threatening”.
He said he threw the punch having already been approached twice by the man.
“I had been trying to walk up there and he kept getting in my face,” he said.
“It's self-defence because I'm not going to wait for him to beat me up in front of my missus," he added.”
He said the punch was seen by security staff, who allowed him into the game.
Robinson had two milkshakes thrown at him during his failed campaign to become an MEP.
He said: “Any man out with their wife should be free to walk the streets without being threatened or abused because of their politics.”
National Police Chiefs' Council spokesman, deputy chief constable Mark Roberts, said UK police have been “informed of an incident” outside the stadium.
“The footage has been passed to our counterparts in Portugal for further investigation,” he said.
In 2011 Robinson was convicted of leading a street brawl involving 100 football fans.
He was sentenced to 12 months community service, 150 hours of unpaid work, and banned from attending football matches for three years.