At least 20 people have been injured, three of them seriously, in an accident at Rio de Janeiro's Sambadrome, the venue for the annual carnival parade by the city's samba schools, BBC reports.

A truck carrying a huge float veered out of control on the wet pavement, crushing a number of dancers and spectators against a fence.

It then changed direction suddenly, hurting more people standing on the other side.

The float's driver is being questioned.

The vehicle, belonging to the Paraiso de Tuiuti samba school, "went into reverse and ended up pressing revellers against the fence on one side," police investigator William Lourenco Bezerra said.

"The main driver of the float mixed with the rest of the samba school and left. But he has already been identified and will be investigated," Mr Bezerra added.

Jaime Cezario of Porto da Pedra samba school told Brazil's Folha newspaper that the float "veered left towards the stands".

"People had nowhere to flee," he said. "Then it went right at high speed, where I was. We had to throw ourselves to one side so we wouldn't be crushed," he said.