Two goals from Luis Suarez either side of a strike apiece from Neymar and Andres Iniesta gave the Liga leaders a resounding victory at the Santiago Bernabeu and prompted a chorus of boos from the home support.
Marcelo, who missed a good chance just after half-time before Iniesta struck Barca's third, says the players must forget the result quickly and fight to claw back the six-point gap to the reigning champions.
"Unfortunately, we failed to do things well. We have to forget this and learn from it," he told Canal Plus.
"We apologise to the supporters, that wasn't the game we wanted to play.
"We'll keep fighting in the way we have to fight."
Madrid managed just 41 per cent possession against Luis Enrique's side and Marcelo accepts that a failure to keep the ball proved their undoing.
"I believe we tried to have the ball but their game is possession. They always had the ball. It's difficult to run without it, it tires you out," the Brazil international added.
"We tried to go on the attack but it wasn't possible."