ISTANBUL (AP) — Cristiano Ronaldo scored a hat trick as a dominant Real Madrid started its Champions League campaign with a crushing 6-1 win over Galatasaray on Tuesday.

Gareth Bale, the world's most expensive footballer, made his Champions League debut for Madrid in the second half of a physical Group B match in which goalkeeper Iker Casillas was replaced by Diego Lopez due to injury. Captain Casillas had not started a competitive match for Madrid since breaking a bone in his hand 238 days ago.

Francisco "Isco" Alarcon opened the scoring after 33 minutes and Karim Benzema went on to strike twice during the second half, when Ronaldo was almost unstoppable.

Umut Bulut scored a late consolation goal for Galatasaray.

"Things could not have started better for us in this season's Champions League," Ronaldo said. "Scoring six goals is really good, it will give us a lot of confidence."

The first shot on target came in the third minute when Casillas was forced to tip Felipe Melo's powerful shot around the post.

Eight minutes later the Madrid 'keeper left the field in visible discomfort after a collision with defender Sergio Ramos. Casillas returned to the field after being heavily bandaged around his ribcage, but three minutes later he was substituted by Lopez.

"Casillas has had bad luck and I hope he will be recovered in three or four days," said Ancelotti, without giving details of the injury.

Lopez was soon in action when he had to save a close range shot by Melo in the 17th after Ramos had been penalized for a foul on Didier Drogba.

Ancelotti said his team had begun a bit nervously.

"The match was good and the result fantastic," he said. "After Isco's goal the match became simpler. We defended well and when Benzema and Cristian Ronaldo have a good night on the counterattack, it's easy, even though winning in this stadium is not easy."

Isco scored the opener after skillfully controlling a long pass from Di Maria and firing his shot in off the post.

Madrid defender Pepe crashed into the back of Drogba just before halftime and the forward left the field clutching his left shoulder in pain. Nordin Amrabat came on for Drogba after the break.

Benzema scored his first after racing forward from midfield and collecting a pass from Di Maria before sending a low shot into the net in the 54th minute.

Ronaldo found the net from close range in the 63rd minute after a goalmouth scramble - just before Bale came on for Isco. Bale promptly sent in a free kick two minutes later which Ramos met with a header that keeper Muslera failed to hold - allowing Ronaldo to pounce on the rebound.

In a hectic ending, Benzema score his second from an assist by Ronaldo in the 81st, Bulut skillfully side-footed Amrabat's long cross past Lopez in the 84th and Ronaldo rounded off the scoring in stoppage time.