RIO: Brazil will enjoy five days of non-stop partying, after Rio de Janeiro's Carnival officially opened.

"King Momo," the portly partier who presides over Carnival, declared the festivities open on Friday as he received the keys to the city after arriving in a limousine flanked by the carnival queen and two princesses.

"For the second straight year I receive the key to the city and declare Rio carnival open," said this year's "Momo," 29-year-old Wilson Dias da Costa Neto, before busting some samba moves as revellers doused him with confetti.

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

But already Rio is awash with hundreds of colourful street parties, known as blocos, each seeking to outdo one another in volume and visual outrageousness, with garish garb and cross-dressing de rigueur.

The countless street groups are a traditional part of Carnival fare stretching 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.

In addition, around a million tourists will descend on Rio, cramming the former capital for the dizzying and decibel-defying spectacle.

Unlike last year, there is no World Cup to look forward to after the festivities, though next year the Carnival will be a prelude to the Rio Olympics, South America's first-ever Games.

AAP