Kaka ushered in Real Madrid's latest "galactico" era as the Spanish club gave the Brazil player a rock star welcome with an unveiling in front of at least 50,000 fans.

After president Florentino Perez introduced Kaka for the first time since his euro65 million ($92 million) acquisition from AC Milan, the 27-year-old playmaker shook his head in delight as fans chanted his name at the Santiago Bernabeu.

"Today is a very special day for me. I'm very happy about this new step in my career at Real Madrid," said Kaka, who will wear the No. 8 shirt.

"Viva Madrid!"

Kaka, who passed his medical earlier on Tuesday, and Cristiano Ronaldo are Perez's major summer signings as he bids to make up for a season without silverware.

"There are no words for today, it's an unforgettable day," Kaka said through a translator.

"All of this is a great honor and responsibility, and not a negative responsibility but a positive one. I hope we can win titles and many victories."

Kaka took the stage in front of a backdrop featuring photos of iconic Madrid players Alfredo di Stefano, Raul Gonzalez, Juanito and Zinedine Zidane.

Accompanied by a young fan donning the first Kaka jersey, he subsequently did a tour of one half of the 80,000-capcity stadium to sign autographs and salute to the pumped-up masses.

"He's very good," said 10-year-old Tony Castano, clad in a Madrid jersey.

"I hope he can help the club win the league and Champions League this season."

Hopes are high at Madrid, which failed to get past the first knockout stage of Europe's top-tier competition for the fifth straight season and lost the league to bitter rival Barcelona, which became the first Spanish club to win the treble last season.

Kaka believes Madrid is in a great position to compete after finishing runner-up last year.

"Madrid has everything in terms of a fascinating team. And it's always been a reference for great players," Kaka said.

"The project is very good. I can't wait to play."

Kaka agreed to a six-year contract with Madrid and will team up with Cristiano Ronaldo, the world player of the year who Madrid acquired for a transfer record 80 million pounds (euro94 million; USD 131 million).

"We're compatible players and I think we'll be able to play alongside without a problem," said Kaka, who had just helped his country to victory at the Confederations Cup in South Africa.

The Brazilian was named the tournament's best player.

Kaka, whose full name is Ricardo Izecson Dos Santos, won a Champions League, Serie A, Club World Cup and two European Supercups in his six seasons at Milan, which he joined from Sao Paulo in 2003.

Kaka was also a member of Brazil's World Cup-winning team in 2002.

Perez has splashed out euro237 million (USD 333 million) this summer - including on Valencia defender Raul Albiol - in a bid to turn the page on a season that was marred by former president Ramon Calderon's resignation over a vote-rigging scandal.

Even Perez was given an ovation when he took the stage, with chants of "Florentino" ringing out for the first time since his initial reign from 2000-06. 

"We believe that this club can become the best club in the world, again," Perez said.

"He is a unique player ... and Kaka is with us."