With 20 minutes remaining, Man United came in waves of finishes.
Antony was the last shooter and found the left post and 2-1 for United was a fact.
The fans erupted into a mighty chant as legendary manager Sir Alex Ferguson jumped to his feet and cheered.
In the fourth additional minute, Robert Lewandowski had a great opportunity to equalize but the ball was cleared before the goal line.
- The match could have gone either way during these 180 minutes.
It ignited at Old Trafford and it was drama until the end, says Viaplay's expert Glenn Strömberg.
After aggressive pressure from an own throw-in a minute into the second half, Bruno Fernandes played the ball in to Fred in the middle, who took down the hard pass and with a quick strike found the right corner and Old Trafford erupted into one big roar for 1-1.
Huge brawl after Fernandes' shot in de Jong's stomach
After just under an hour, there was a huge scuffle with all the players involved after Barcelona's Frenkie de Jong fell slightly in a melee and then Bruno Fernandes shot a ball hard into his stomach from close range.
It ended with Fernandes and Franck Kessie each receiving a yellow card.
Barcelona led after the first half with 1-0.
Robert Lewandowski curled in a penalty that David De Gea was on at the right post but failed to clear.
Victor Lindelöf and Anthony Elanga remained on Man United's bench for the entire match.
Sir Alex Ferguson, 81, had earlier this week had dinner with current coach Erik ten Hag.