With a growing record of solving or easing Rio's urban problems, authorities want to fix something else once considered unfixable - the chaotic, and increasingly popular, Carnival street parades, known as blocos, attended by millions.

Singing, drinking, dancing and revelry are all fine, says Rio de Janeiro Mayor Eduardo Paes. Urinating in the streets is not.

Since the pre-Lenten samba festival season officially began on January 20, more than 800 men and women have been arrested for public urination, an offense that has been rarely enforced before. For many Cariocas, as the residents of Rio are known, the crackdown seems unrealistic.

"Nobody likes the mess, but I don't know how you can punish someone for doing something they must do," said Joao Pimentel, author of the 2002 book "Blocos," about Rio's street Carnival.

In 2011, a judge threw out a public urination charge saying a person's biological needs trumped Rio's public-order campaign.

For Pimentel, it's not just about manners, it's impractical.

The Bola Preta (Blackball Parade), on Saturday attracted 2.2 million people. Many were packed shoulder-to-shoulder in 35 Celsius (95 Fahrenheit) heat 80 across on narrow, downtown streets.

There were only 400 chemical toilets scattered over dozens of blocks. Meanwhile, vendors with cold, 3-real beers were rarely more than a step or two away.

"There's too much beer, too many people and never enough toilets even if you could get to one in time," Pimentel said.

Those 400 toilets, one for every 5,500 people, would have begun overflowing if only about 1 in 8 of the Bola Preta revellers used one, said Joao Aveleira, a medical doctor and Carnival enthusiast who founded the Suvaco do Cristo (Christ's Armpit) Carnival group 25 years ago. The parade takes place under the outstretched right arm of Rio's famous Christ the Redeemer statue.

While Aveleira, Pimentel and others complain about the lack of toilets, there are more toilets this year than in the past. A decade ago side streets near Bola Preta would be awash with an inch or more of urine and smelled foul for weeks.

CARNIVAL FOR ALL

In the past, Rio merely tolerated the street Carnival, focusing on the more famous and structured samba school parades at Rio's Sambodrome stadium, Aveleira said.

At the Sambodrome, costumes and the right to participate cost hundreds to tens-of-thousands of dollars, samba school budgets run to the millions, spectators are charged for admission and performances are timed and rigorously judged. The street carnival is free and open to all.

Under Paes, and his predecessor Cesar Maia, who tried to encourage healthy living by ending the tradition of selecting an obese man as Carnival king, blocos have grown but maintain their informality.

"They've finally realized that street Carnival is big business, Rio's main event," said Aveleira.

"People come to Rio because our Carnival is democratic. In Salvador, there is a strict hierarchy: the rich in their boxes, the middle class paying to dance inside a roped off area and the poor on the edges," Aveleira said. Salvador and Rio are the country's two biggest Carnival capitals.

Rio's efforts to clean-up Carnival are part of a wider effort to get Cariocas to behave more civilly. Paes has set up public order units that tow away illegally parked cars and remove vendors from crowded city streets.

He has also worked with Rio's state-level authorities to institute drunken driving checks. which have cut road accidents 32 percent since 2009, according to Brazil's Health Ministry.

The Choque de Ordem (Order Shock) campaign seems to be working at Carnival too. Rio's orange-clad street sweepers, who swoop in with brooms, dustbins and high-powered water jets after every street parade, collected 35 percent less garbage after Bola Preta this year than last.

"We've had as many as 50,000 people at our events, and there were years where there were fights and gunshots. Not anymore. This is supposed to be fun, free but not chaos," said Aveleira.

This year Suvaco do Cristo, sold 800, 25-real T-shirts to pay for the event and 35,000 revellers showed up February 12 for the parade.

On Tuesday 200,000 people danced along Ipanema beach at Afroreggae. Another 60,000 turned up in Flamengo Park at a Sargento Pimenta (Sergeant Pepper) event where Beatles songs are sung with samba percussion.

Rio arrested 77 at Afroreggae for public urination.

"Like any other thing that grows up, you have to start acting more responsibly," Pimentel said. "The restrictions and efforts by the city are good, but we still need more toilets."

(Reporting by Jeb Blount; Editing by Jackie Frank)

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