Paris Saint Germain coach Unai Emery believes he was let down by German referee Deniz Aytekin as his team slumped to a sensational 6-1 defeat to Barcelona on Wednesday in the Champions League.

Despite taking a 4-0 first leg lead into their Camp Nou last-16 return, and grabbing a crucial away goal from Edinson Cavani on the hour mark, PSG shipped three goals in seven minutes at the end of the game as Barcelona completed the greatest comeback in the history of the tournament.

Just when Barca looked down and out, trailing 5-3 on aggregate, Brazilian star Neymar restored hope when he curled home a sensational free-kick high into the top corner.

And as the match entered stoppage time, Barca were awarded a second controversial penalty by the referee when Luis Suarez tumbled under a challenge form Marquinhos.

Neymar sent goalkeeper Kevin Trapp the wrong way.

The Brazilian then showed a cool head in the fifth minute of injury time when he cut inside before chipping into the box for substitute Sergi Roberto to extend his right leg just enough to beat Trapp and leave PSG floored.

"The truth is we have let a huge opportunity get away and we are aware of that. In the first-half it was more our fault than them playing well," said Spanish coach Emery whose side trailed 2-0 at the break.

"In the second-half it changed. The penalty (first penalty converted by Lionel Messi for 3-0) got them off to a great start, but I was already calmer because I could see the team was responding better, were better positioned on the pitch and could do damage.

"We had chances to make it 3-2 and then the refereeing decisions, I don't know if they were right or not, but for sure they damaged us. Then in the last two minutes we lost everything we had recovered in the second-half.

"Barcelona are capable of this in their stadium. In the last few minutes they played all or nothing and they have beaten us."

PSG's Qatari president Nasser Al-Khelaifi was stunned by the turnaround.

"It's a nightmare for everyone. Three goals in seven minutes is very difficult. Everybody is upset but now is not the time to talk about it," he said.

French sports daily newspaper L'Equipe was in no doubt about the implications of the defeat.

"Inqualifiable" ("Unspeakable") was its front page headline for Thursday with its reporter claiming the entire "PSG project" was threatened by the "humiliation".