Ryan Giggs headed the hosts ahead before Rooney grabbed centre stage, firing in his first goal from Ruud van Nistelrooy's precision pass.

He rifled in a second from 20 yards out and scored a second-half free-kick.

Marcio Nobre and Tuncay twice reduced the deficit for the Turkish side, but late goals from Van Nistelrooy and David Bellion made sure of the result.

United now move top of Champions League Group D, and despite the emphatic scoreline, Sir Alex Ferguson will be keen to address some of the defensive lapses that could have cost his side dear.

With Rooney making his first appearance in a United shirt since his £27m move from Everton, all eyes at Old Trafford were on the England striker.

And the 18-year-old did not disappoint, instinctively laying the ball off for Bellion on the right only for the Frenchman to produce a poor cross.

Despite not having kicked a ball competitively since limping out of England's Euro 2004 quarter-final defeat to Portugal, Rooney showed that he had not lost his touch.

His mere presence lifted United, who were also bolstered by the return of Gary Neville at right-back.

While there was a definite heavyweight feel to United's attack and defence, the home side's midfield appeared vulnerable.

Roy Keane was rested, John O'Shea was on the bench, while Paul Scholes was out injured so in came Kleberson and Eric Djemba-Djemba.

Yet the pair acquitted themselves and Kleberson had a hand in United's first.

Giggs found the Brazilian on the left wing and he bided his time before whipping in a delightful cross for Giggs to send a glancing header low into the left-hand corner.

The early goal settled any United nerves and paved the way for Rooney to stamp his mark on the match.

And it did not take long for the youngster to find the back of the net.

United broke on the counter-attack, the ball working its way to Van Nistelrooy, whose defence-splitting pass found Rooney who finished with aplomb from the edge of the area.

It was the perfect start for Rooney, but things were to get better for United's new boy when he hammered home an unstoppable shot into the bottom right-hand corner.

Gabriel Heinze, who was again impressive at left-back, raced forward on another United counter-attack, passing the ball to Giggs who laid off the ball to Rooney.

The striker had only the goal in his sights and his radar did not let him down.

United left the door ajar for Fenerbache, who reduced the deficit after the restart when Nobre capitalised on poor defending to send Alex's corner past Roy Carroll.

Rooney rode to United's rescue seven minutes later, when he restored the three-goal lead with a brilliantly-executed free-kick which he curled past Rustu Recber.

It was the first hat-trick of Rooney's senior career and capped a simply breathtaking display of attacking football from the teenage prodigy.

Yet United's newest star was guilty of inexplicably conceding a corner from 40 yards out from which Fenerbache scored their second.

The United defence failed to clear their lines, and while Carroll produced an excellent point-blank save, he could do nothing about Tuncay's shot from the edge of the area.

It was the only real blip on a memorable first appearance for Rooney in a United shirt and in the Champions League.

Rooney's strike partner Van Nistelrooy struggled to find his rhythm, but the Dutch master was not to be upstaged.

He continued his amazing European goalscoring run by lashing in United's fifth after brilliantly controlling Darren Fletcher's cross from the right on his chest.

But United were not finished and Mikael Silvestre's long ball out of defence was flicked on by the outstanding Rooney for Bellion to run onto and complete the rout.

Man Utd: Carroll, Gary Neville, Heinze (Phil Neville 81), Ferdinand, Silvestre, Bellion, Djemba-Djemba, Kleberson, Giggs (Fletcher 62), van Nistelrooy (Miller 81), Rooney.

Subs Not Used: Ricardo, Ronaldo, Smith, O'Shea.

Booked: Heinze.

Goals: Giggs 7, Rooney 17, 28, 54, van Nistelrooy 78, Bellion 81.

Fenerbahce: Rustu, Baris, Luciano, Ozat, Akyel (Akin 61), Balci, Aurelio, Marcio Nobre, Sanli, Alex, Van Hooijdonk.

Subs Not Used: Demirel, Rodriguez, Yozgatli, Hacioglu, Turaci, Sahin.

Booked: Aurelio, Balci.

Goals: Marcio Nobre 47, Sanli 60.

Att: 67,128

Ref: Frank De Bleeckere (Belgium).