With whimsical floats, thundering drum sections and legions of performers in fanciful, flesh-flaunting costumes, 12 samba schools are competing for the coveted title of carnival champions across two nights of epic booty shaking.

Entering the parade venue "gives me goosebumps every time," said Debora Moraes de Souza, a 53-year-old doctor who grew up in the impoverished neighborhood of Sao Goncalo and has been parading for a decade with its samba school, Porto da Pedra.

"You get to the end and you say, 'Oh, it's over already? I want more!' Everyone's jumping, everyone's happy."

Rio has already been celebrating carnival for weeks with colorful, free-for-all street parties known as "blocos."

Sunday's and Monday's parades are the climax: sumptuous festivals of color and sound that last all night and into the next day.

A capacity crowd of 70,000 spectators cheered from the packed stands of the Sambadrome stadium, the city's purpose-built parade venue, with millions more expected to watch live on TV.

But there is more to carnival than all-night partying.

The samba schools are rooted in Rio's impoverished favela neighborhoods, and each parade tells a story, often dealing with politics, social issues and history.

This year's parades include homages to little-known heroes of Afro-Brazilian history and a celebration of the Yanomami Indigenous people, who have been ravaged by a humanitarian crisis blamed on illegal gold mining in the Amazon.

The school behind that parade, Salgueiro, linked the plight of the world's biggest rainforest to the fight against climate change, in which the Amazon's carbon-absorbing trees play a vital role.

"We're here to show everyone what's happening in the Amazon," said dancer Kevin Rodriguis, 22, after being extracted from atop his float by a crane at the end of the parade.

"The Yanomami are in crisis, there's lots of deforestation, people and animals are dying, trees are being burned."

- Mishaps happen -

Each samba school has 60 to 70 minutes to dazzle its way down the 700 meters of Marques de Sapucai, the avenue through the concrete carnival parade temple designed by modernist architect Oscar Niemeyer.

A jury judges each down to the minutest detail, with potentially devastating fractions of points deducted for being out of sync, running over time or lacking flair.

Porto da Pedra was set to lose precious points after suffering a pair of float mishaps -- not uncommon at the parades.

In one, a piece of a float broke right in front of the jury. In the other, a float got snagged on a metal security grate, dragging it and injuring a woman.

She was treated for cuts to her leg and released, the city health department said.

Pulling together a show with more than 3,000 performers and a fleet of seemingly gravity-defying floats is no easy feat.

The samba schools spend the entire year preparing -- and often face a down-to-the-wire race to get ready.

- 'From the heart' -

Alexandre Reis, a 52-year-old electrical technician from the Beija-Flor school, was heading up a team rushing to fix a last-minute problem: the lights on one side of their float stopped working.

Reis has been on hand to handle just such emergencies for the past 23 years.

"It's a very complex job. The lighting demands a lot of technical expertise, because (the floats) are like a moving, open-air stage," he said.

But "I do this from the heart," he added. "We sweat and bleed because we love the school."

The parades were particularly political under far-right ex-president Jair Bolsonaro, who faced accusations of authoritarianism, racism, environmental destruction and disastrous mishandling of Covid-19 -- all fodder for the samba schools during his 2019-2022 presidency.

The overall tone is less politically charged since veteran leftist Luiz Inacio Lula da Silva returned to the presidency in January 2023.

Invented a century ago by the descendants of African slaves, samba is one of the great symbols of Brazilian popular culture, and of Rio.

Today, carnival is big business for Rio: the party is expected to generate 5.3 billion reais (more than $1 billion) in revenue this year.