The world's most celebrated Carnival parades have kicked off, with thousands of feather-clad dancers, wildly costumed marchers and musicians defying a tropical deluge at Rio de Janeiro's Sambadrome.

The skies opened up about an hour ahead of the start of the all-night-long extravaganza, drenching revelers and flooding streets near the Sambadrome.

Participants wore plastic rain slickers that they discarded at the last possible moment before taking to the runway.

Six samba schools were parading on Sunday night. Each of the groups has 82 minutes for its floats and thousands of performers to parade. Six other schools will perform on Monday, with all 12 groups vying for the title of the year's best.