RIO DE JANEIRO (AFP) - The glittering revelry of Rio de Janeiro's carnival descended into mayhem again on Tuesday (Feb 28) when a freak accident struck a float for the second time in two nights.

Twelve people were injured when the top level of an elaborate three-storey float collapsed under the weight of the brightly costumed dancers atop it, crashing onto the crowded platform below.

Police and firefighters rushed to evacuate scantily clad victims from the packed Sambadrome, the elongated stadium that hosts the carnival's headline event: the elite samba school championship parade.

It has been a rough year for Rio's legendary carnival. Another float swerved into the crowd and injured 20 people late on Sunday, on the championship's opening night.

But in both cases organisers decided the show must go on.

Tuesday's accident occurred around 3am, as members of the Unidos da Tijuca samba school strutted their stuff atop an enormous float dedicated to "New Orleans, the City of Jazz" - the school's theme for the year.

Slowly, the top floor of the faux-brick float - designed to look like a building in the New Orleans French Quarter - keeled to one side and collapsed.

"I was on the platform of the float, to the left side, and suddenly I saw the right side collapse," dancer Raissa Ribeiro said through tears just after exiting the stricken float. "People fell into the float's interior and some of them got stuck inside the metal structure."

As ambulances rushed to the scene, police struggled to push back dozens of photographers who flocked to snap pictures.

After more than half an hour of chaos, the damaged float was allowed to finish the 700m parade through the Sambadrome, rescue workers still on its roof. That drew applause from the crowd, even as terrified dancers cried on the sidelines.

The carnival's all-time winningest samba school, Portela, then took the floor as if nothing had happened.

"These things happen, but there should be a security review. Carnival's a serious deal. We work hard all year, so when something like this happens, everyone's angry," said Washington Luis, a member of Unidos da Tijuca.

Sunday night's accident occurred when the Paraiso do Tuiuti samba school's float careered out of control in the rain.

It drove too close to a fence at the entrance to the competition piste, trapping several people. Three were seriously injured, including a news photographer who had her leg badly crushed.

But despite the pools of blood left on the rain spattered ground, the party soon got back in swing.

The head of the carnival's main organizing body, the Independent League of Samba Schools, called it a "coincidence" that two accidents hit the event in two nights.

"They each had different characteristics," the official, Jorge Castanheira, told Brazilian news site G1.

The organisers' gritty resolve to carry on with the festival of sparkling costumes, near-naked dancers and pulsating beats seemed fitting for Brazil, where this edition of carnival is an escape from the worst recession in a century and a particularly ugly year in the country's corruption-stained politics.