The lad was one of 3,500 Newcastle United supporters penned in after the final whistle in Lisbon on Thursday.
As tempers rose at the delay, mobile phone footage captured the moment one cop in full riot gear began swinging his baton at fans – some children.
The film shows the lad fall backwards from the blow before being quickly surrounded by protective adult fans.
One shouts, “He’s a f****** child, man”, another points to his head to indicate where the boy was hit and boos erupt.
Other riot officers waded in after the cop appeared to react to being touched on the head.
Newcastle fan Jimmy Archer, 47, said: “They were whacking everybody at random. I got hit twice and all I had said was, ‘Stop’.
“After we’d left the ground people talked of a young lad being hit. It was crazy. It all ended as soon as it began and there’d been no reason for it. Nobody was throwing punches or anything.”
Fan Michael Lowdon said: “The police took no account of the fact we were penned in.
“There was pushing going on and I think the police just snapped and brought out the batons. It was really scary.”
It is thought the unidentified boy went with his dad to the game against Benfica, which Newcastle lost 3-1.
After the match away fans were allowed to swap scarves and shirts with Benfica supporters but were then kept back by police.
Later, fans of the Premier League club went on social network sites to tell of the attack. One claimed he witnessed the lad being hit.
Newcastle United said: “We encourage the boy’s father to make a formal complaint to the Portuguese police.”
Match commander Valente Pinho said: “We sent officers in to avoid any trouble between fans. I think we did it well.”