All participants and the 75,000 attendees expected each night are required to present proof of vaccination.
Each group in the competition has 60 to 70 minutes to tell a story in music and dance, evaluated on nine criteria by the jury.
The reigning champions, Viradouro, paid tribute to Rio's epic 1919 carnival -- the first celebrated after the devastation of another pandemic, the Spanish flu.
"No sadness can withstand so much joy," goes their samba theme song.
"It's a love letter to carnival... (about) coming back from a health crisis, taking off our masks and coming out stronger than we were before," said U.S. expatriate Leslie Mercado, 48, at the parade finish line, after being extracted by crane from the top of a giant crystalline float that looked like a shimmering chandelier.
Other schools picked themes charged with social messages, with Brazil facing divisive elections in October expected to pit far-right President Jair Bolsonaro against leftist ex-leader Luiz Inacio Lula da Silva.
Of the 12 schools, eight chose themes dealing with racial injustice or Afro-Brazilian culture and history, loaded issues in a country where the current president has faced frequent accusations of racism.
"Brazil still suffers from racism. Nothing has changed. Slums, hunger, poverty... they have a color here: black," said Aristoteles Silva, 52, parading as a warrior for samba school Salgueiro, whose theme song was an anti-racist anthem entitled "Resistance."
"I'm hoping the election will bring a total change."
Carnival should also provide some needed relief for the pandemic-battered economy.
Beyond the swirl of floats, feathers and barely covered flesh, carnival is big business, moving an estimated four billion reais ($800 million) and creating at least 45,000 jobs.
But street vendor Maria Vitoria Souza, 18, who was selling drinks outside the parades, said sales "could have been better."
"Carnival's still not back 100 percent, because there are no 'blocos'" -- massive carnival street parties, which city authorities nixed this year.
Participants were just happy the party was on.
"We've had two years of so much darkness in the world," said Latino Suarez, 45, who traveled from Sao Paulo to parade.
"Brazil without carnival isn't Brazil. It's part of who we are."
