Rio has plunged into five days of hedonistic revelry 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, moon-faced 27-year-old chosen as this year’s Momo, arrived on Friday 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 centre’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 sequined dancers that cars will for once be forced to take a back seat.

Samba school dance during the final technical reheasal at the Sambodromo in Rio de Janeiro, Brazil. Photo: AAP

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-metre Marqus 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 Cordo 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 – including Sargento Pimenta (Sergeant Pepper), who will Monday pound out Beatles music to a samba beat.

Adding to the millions of local revellers, the city hall expects 918,000 tourists to throng the jam-packed streets – some two per cent more than last year – with revenues forecast at around $US400 million ($A448.03 million).