(AFP) - The glittering revelry of Rio de Janeiro's carnival descended into mayhem again on Tuesday 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-story 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 Sambodromo, 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 Sunday, on the championship's opening night.

But in both cases, organizers decided the show must go on.

- 'Stuck inside' -

Tuesday's accident occurred around 3:00 am (0600 GMT), 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 group's theme for the year.

Slowly, the top floor of the faux-brick float -- designed to resemble a building in the Louisiana city's French Quarter -- tipped 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 700 meter (yard) parade through the Sambodromo, 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.

City transport officials vowed to step up safety checks next year.

- 'Life must go on!' -

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

It swerved toward a fence at the entrance to the competition area, trapping several people. Three were seriously injured, including a news photographer who had her leg badly crushed.

Three victims from that accident remain hospitalized, including one on artificial respiration. One remains hospitalized from Tuesday's accident.

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.

Another safety scare nearly turned ugly in the early hours of Tuesday. A gold-clad dancer from the Mocidade samba school plunged 1.5 meters (five feet) when her elaborate silver platform collapsed. But she escaped without injury.

The organizers' 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.

After the opening-night accident, the announcer had begun the second night's festivities with a tribute to the victims.

Then he exhorted the crowd to set aside such troubles and cut loose.

"Life must go on!"

© Agence France-Presse