Referee Kevin Friend pointed to the spot at the Stadium of Light after John Mensah fouled Jay Spearing, despite the tackle appearing to take place outside the box.
Dirk Kuyt converted to inspire the Reds' 2-0 win, with Luis Suarez
"The referee was 10 yards from the incident and the incident clearly took place outside the box," Bruce told Sky Sports News.
"There wasn't a second challenge that tumbled him into the box. If you look at John Mensah when he goes to ground, he's a yard outside when he makes contact.
"It was a mistake from him, he tried to rectify the mistake.
"The referee, who's 10 yards away, gives a free-kick for at least 20 seconds. Then obviously the linesman jumps in, from 80 yards away, and gives a decision which obviously was wrong.
"I'm all for getting the decision right. If that decision had been right then you've got no question.
"It's down to big decisions again which, unfortunately for me, they've got wrong."
Bruce believes Liverpool's penalty was the catalyst to their victory, adding: "For me that was the turning point of the game.
"We were well-beaten after that, we have to hold our hands up."