By Stuart Grudgings

RIO DE JANEIRO, Feb 22 (Reuters) - As the sun sets behind a church in a hillside Rio de Janeiro square, Rodrigo Correa adjusts his nun's habit and mini-skirt before bounding into a throng of revelers following a booming mobile sound system.

It is 6 p.m. and another Carnival "bloco," the neighborhood street parties that are the lifeblood of Rio's annual feast of hedonism, is getting into its stride.

"This is what Carnival is all about -- happiness, costumes, the blocos, without discrimination, full of women," Correa, a 53-year-old economist shouted as the Samba band struck up on Saturday in the historic Santa Teresa district. "It's the real Carnival of Rio de Janeiro."

For all the millions of dollars and blanket television coverage invested in the grand parades at Rio's Sambadrome, the beating heart of Carnival remains its free-to-enter blocos, where a Samba band and a jury-rigged sound system are enough to sustain a thousands-strong party for hours.

Some of the hundreds of blocos have histories going back almost a century and command a loyalty from Cariocas, as Rio's residents are known, rivaling that reserved for soccer teams and the Samba schools that parade through the Sambadrome.

The blocos have something for everyone. There is a bloco for dogs, for children, for men who like to dress as late Brazilian singer Carmen Miranda wearing hats of plastic fruit, and another where dressing as nuns is encouraged.

The informal blocos have expanded at a dizzying pace in recent years as the larger ones spawn break-off groups seeking their own place in the Carnival sun.

Some start early to beat the rush. By 11 a.m. on Saturday, several hundred thousand people had crammed Rio's historic center to welcome the Cordao da Bola Preta (the Black Ball bloco) party, one of the most popular and historic.

TRADITION

Oswaldo Juneor, a portly 53-year-old salesman who was drinking beers with a group of friends decked out smartly in black-and-white with straw hats, said he had been coming to the Black Ball bloco for nearly 30 years.

"This bloco is 90 years old so there's a lot of old timers who still come here today," he said. "Some people go to many blocos but I only come here. This is a tradition."

Others come for more transparent reasons. Nearby, two young men lined up for a kiss with an accommodating woman as the streets began to fill with discarded beer cans.

For "Amanda" Carraro, 44, a designer who normally goes by his male name, it was a chance to show off his fashion -- a svelte black-and-white spotted dress topped by a matching hat that could have graced the Kentucky Derby.

"I normally go to the gay parades in (Rio beach district) Ipanema but I'm giving this one a try for the first time," Amanda said, before flouncing off with another man in drag.

In Brazil's sometimes conservative society, the four-day pre-Lenten festival is a time when social mores are cast aside. Men often take the chance to show their female side.

Vinicius Adelino, 26, a physical education teacher, couldn't remember if he had drunk nine or 10 beers but said he felt comfortable in his fairy wings and fake breasts.

"It's the one time in the year when men can dress up as women and vice versa. I like women," said Adelino, who had plans to attend another three or four blocos during Carnival.

By midnight in Rio's Lapa nightlife district, it was clear that the early-morning blocos had extracted their price on partiers. Ben from California, wearing a "Black Ball" hat, was slumped against a beer vendor's cart.

Behind him, another bloco was starting.

(Editing by Bill Trott)

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