Carnival King Wilson Neto (2nd R), Carnival Queen Leticia Guimaraes (R), Carnival Princess Clara Paixao (2nd L) and Gracielle Chaverinho pose for pictures during the handing over of the ceremonial key to the city, at Cidade Palace in Rio de Janeiro. REUTERS/Sergio Moraes

Rio - Rio plunged into five days of hedonistic revelry Friday as the city's legendary carnival opened, with King Momo taking the keys of the city and declaring joy by decree.

Wilson Dias da Costa Neto, a smiling, moonfaced 27-year-old chosen as this year's Momo, arrived by limousine flanked by the carnival queen and two princesses to assume his festive reign.

“In the name of the prefecture and as the monarch of the world's largest street festival I decree joy,” Momo beamed.

Mayor Eduardo Paes joked as he passed him the keys: “I hope you'll be able to sort out the city center's problems with automobile traffic!”

In reality, within hours the city's streets will be so clogged with a gyrating mass of samba drummers and sequinned dancers that cars will for once be forced to take a back seat.

This year's festivities are set to be even more frenzied than usual as Brazil prepares to host the World Cup for a month from June 12.

A dozen samba schools will go compete Sunday and Monday in the Special Parade which will draw around 70,000 spectators to the city's fabled Sambadrome.

The schools, each comprising thousands of performers, will parade along the 700-meter (yard) Marqués de Sapucai avenue leading to the Sambadrome designed by celebrated Brazilian architect Oscar Niemeyer.

But countless street groups or 'blocos' will also participate in a tradition that goes back to the early 18th Century. They will be led by the Cordão da Bola Preta, Rio's oldest group founded in 1918 which will gather an estimated 1.8 million people.

Dozens of other blocos will parade in different parts of town Äincluding Sargento Pimenta (Sergeant Pepper), who will Monday pound out Beatles music to a samba beat.

Adding to the millions of local revelers, the city hall expects 918,000 tourists to throng the jam-packed streets Ä some two percent more than last year Ä with revenues forecast at around $400 million.

They will more than make up for those residents who pack their bags at this time of year to escape the constant din.

Sporadic public protests in recent months against political corruption and the multibillion cost of the World Cup have prompted police to step up security in the city, which will host seven World Cup matches, including the July 13 final.

Ahead of the official opening, Thursday evening saw the Ocupa Carnaval pressure group hold a spoof mini-carnival while the Banda da Rua do Mercado bloco held a topless march with actress Ana Paula Goncalves Nogueira, suitably unrobed, at its head.

Some in the Ocupa Carnaval group held aloft giant puppets representing city rulers, their pockets stuffed with money; others linked arms with the tentacles of a huge black model octopus representing the long reach of corrupt politicians.

“But tomorrow's carnival and that is sacred. There won't be protests Äit's a valve to let out the frustration with politics,” one local, Reinaldo Alves dos Santos, told AFP.

Throughout, the non-stop samba beat will propel the huge gyrating crowd along, each bloco's line of revelers extending for several hundred meters (yards) down the street, almost merging with neighboring groups.

By Sunday night, some four million people will have let their hair down and many will carry on through Tuesday's official close and beyond.

Rio is, however, by no means alone in its passion for carnival.

The northeastern city Salvador de Bahia, the cradle of Brazil's African heritage, also hosts huge blocos which since Wednesday have been parading in the paved streets of the city's historic old town Pelourinho.

Further north, Recife and neighboring Olinda host thousands of dancers performing the “frevo”, a rhythm inspired by martial arts-cum-dance capoeira, led by the “Galo de Madrugada” (morning cockerel) troupe.

Business hub Sao Paulo, while more staid than Rio, also has a sambadrome and will host its own parades on Saturday, including one honoring retired soccer world champion and all-time World Cup top scorer Ronaldo.

In Rio, football gets a further nod with the Imperatriz Leopoldinense samba school paying tribute to 1980s star Zico.