A performer from the Sao Clemente samba school parades during carnival celebrations at the Sambadrome in Rio de Janeiro.

Rio de Janeiro's samba schools are vying for the title of the year's best in an over-the-top, all-night-long parade at the city's iconic Sambadrome.

The powerful percussion sections, larger-than-life floats and nearly nude samba dancers of six top-tier schools paraded on Monday night. Six other schools, including reigning champion Unidos da Tijuca, paraded on Sunday.

Results are announced later in the week.

The Sambadrome parades are the highlight of Rio's six-daylong Carnival extravaganza, which also includes hundreds of block parties which attract hundreds of thousands of costumed revellers.

Over the next two nights, 12 schools will perform before a global television audience in the highlight of the five-day extravaganza, a fierce contest watched with the same fervour as football matches in this soccer-mad nation.

The school floats are packed with dancers wearing huge headgear, feathers, sequins, body paint - and little else.

The VIP guests at Rio's 72,500-seat open-air Sambodrome included the sultry 26-year-old American actress Megan Fox, of Transformers fame, who was hired to promote Brahma Beer, a popular Brazilian lager.

“I'd love to be Brazilian. I wish I had the Brazilian butt,” Fox told reporters. “I'll stay until bedtime. It's an opportunity of a lifetime.”

Other celebrities include TV reality show star Kim Kardashian and boyfriend rapper Kanye West; Italian bombshell Monica Bellucci and her French husband, actor Vincent Cassel; and Men In Black star Will Smith, who earlier visited the favela (shanty town) of Vidigal in southern Rio.

Carnival wraps up on Ash Wednesday. - Sapa-AP-AFP