The "nuns" are all followers of the Carmelitas, a group started in 1991 by friends who gathered for soccer and drinks just outside a convent of Carmelite nuns. Jokes about the sisters escaping to join the party gave rise to the band, which parades twice: at the beginning of Carnival, when the nuns supposedly escaped the convent join the fun, and on the last day, when they returned to their cloistered existence.

"We’re keeping the tradition, remembering the first nuns who jumped the fence," said Eliete dos Santos, 25, who was out with five other costumed "sisters" as the partying began Friday.

While the public face of Rio’s Carnival is the famed two-day parade of samba groups, which can each spend more than $5 million on extravagant costumes and floats, its heart lies in these roving groups of irreverently costumed, mostly inebriated partiers who create a free, open-to-all street Carnival. Their cavorting is likely to hit a fever pitch Saturday.

A revival over the past 10 to 15 years has given the roughly century-old tradition of street bands a new swagger. This year, 424 of them registered with the mayor’s office. Countless others, some little more than a group of good friends with a band and some beers, don’t even bother with the legalities.

About three weeks before Carnival’s official kickoff, they start parading around town, tying up traffic and playing traditional tunes or their own, wacky theme songs composed to make fun of the year’s news, politicians, celebrities, or anyone who strikes their fancy.

This being Carnival, there are only the loosest rules. Generally, they congregate on a street corner, a bar or a square. Once their following swells to a sufficiently animated band of revelers, they perform a short circuit around the neighborhood, drawing along the dancing masses.

Their names reflect their idiosyncrasies. Some carry monikers that point to their geographical origin in the city: "Suvaco de Cristo," literally translated as "The Armpit of Christ," is based in a neighborhood more or less underneath the outspread arms of Rio’s Christ the Redeemer statue.

Many of these roving street bands go for laughs with impossible to translate names that make not-too-subtle references to two favored Carnival pursuits: making out and drinking. "Come with me, I’m easy," ”I’ve stopped drinking, but I haven’t stopped lying" and "Poke her and she’ll jump," are among the tamer examples.

For the Carmelitas, nuns reign even though all are welcome.

Marcelo Carvalho, 34, dressed as a protesting Egyptian in a tunic and sign saying "Down with the mummies" tried to talk some of them into a change of habit.

"Islam is where it’s at," he told the costumed nuns. "After all the scandals, you can’t shame the Catholic Church anymore. Try a new religion!"

The "Carmelitas" band entices thousands through the steep, cobble-stoned streets of Santa Teresa, one of Rio’s most picturesque neighborhoods.

Some bands call out to small but specific crowds.

One group devoted to pets and their owners leaves in the early morning and promenades along Copacabana beach. Its sound truck, topped by a giant inflatable puppy, draws a family-friendly crowd. The animals range from dogs in fairy wings, party hats and ballet tutus to more exotic pets like a rooster in a polka-dot clown suit.

Rita dos Santos has brought her white poodle every year since the band’s inception nine years ago.

"For every day of carnival she has a costume," she said, holding up the fluffy white pup in a burlesque outfit of black leather and lace.

"Me Beija que sou Cineasta" — translated as "Kiss me, I’m a filmmaker" — appeals to those in the industry and their fans. "Imprensa que Eu Gamo," which translates loosely as "Press me and I’ll fall for you," brings together journalists and friends.

The lyrics of their theme song always play off the year’s news: This time, WikiLeaks, Hosni Mubarak’s fall in Egypt, and the election of Brazil’s first female president were all fair game.

"Come uncover my scoop, and read between my lines. … Use and abuse your thesaurus, and fall into samba, my friend," participants belted out the chorus in a marketplace as they drew followers.

"The dancing is not their specialty, but the conversation is great," said Erica Paim, 21, a college student who has donned the press group’s black-and-red colors for three years in a row. "Everyone knows everyone, and everyone drinks."

Others have become Rio institutions.

The "Banda de Ipanema" was founded in 1965, in the wake of a coup that established a military dictatorship in Brazil, and prizes irreverence above all. It draws cross-dressers galore: men in elaborate fruit-topped hats and halter tops a la Carmen Miranda together with police officers in short shorts and strings of condoms as ammo belts.

It’s the gayest street band in town, Alci Bueno, 41, said as it met near an Ipanema street that concentrates many of the area’s gay bars. Bueno comes from Niteroi, a town across the bay from Rio, to join the fun.

"It’s a Carnival classic — traditional and eclectic," he said of Banda de Ipanema. "You find all kinds of people — anything you can imagine. Anything — so be careful!"