The 2010 Rio Carnival got under way yesterday under a zero-tolerance campaign against petty crime and disorder.

The anything-goes atmosphere of the "world's greatest party" is subject to the city mayor's year-old crackdown on debauchery.

In the first carnival since Rio won its bid to host the 2016 Olympics, those who drink too much beer at giant street parties and want to use gutters as toilets - always tolerated in the past - are out of luck. Police have already arrested nearly 100 such lawbreakers.

Beaches no longer resemble full-service bazaars with greasy snacks delivered on command under a sprawl of rented umbrellas. Beach football is only allowed after 5p.m.

And something else is missing. Rio's world-famous waterfront pick-up club for legal prostitutes has closed to make way for a museum.

The changes were getting mixed reviews even before the party officially started.

"The plan is doing the impossible: making Rio square," said Marcus Paulo Reis, a 36-year-old businessman sipping beer at lunch this week in the beachside Arpoador neighbourhood. "They're trying to get rid of the grit that gives Rio its flavour."

Violence is still the city's biggest security concern, with at least seven suspected drug traffickers and a policeman killed on Thursday in a shootout in a northern slum.

There appears to be a silent majority glad to get some peace on the beach and during this week's party as international headliners poured into Rio, including Madonna, Beyonce, Alicia Keys and Paris Hilton.

Much like New York City's government did in the 1990s, Mayor Eduardo Paes wants to end Rio's general lawlessness with a zero-tolerance campaign he calls "shock of order".

The headlines have come non-stop for a year: shock of order on the beaches, shock of order on public transportation, shock of order on illegal billboards.

As he handed the key to the city over to the Carnival King Momo - who rules the city until Ash Wednesday - Mr Paes made it clear who really was in charge. He stood before journalists holding a sign underscoring his anti-urine campaign that read "Come on, don't pee here, OK?"

Annatolio Isidoro, a dapper 77-year-old dressed in a tan, linen suit and straw bowler hat who serves as the official "Samba Citizen", said the rules would make the carnival better.

"We've long needed some organisation to better show off our great party to the entire world," he said at the handover ceremony.

But the effort has left a bad taste in the mouths of some Cariocas, as Rio residents call themselves, especially those who make their living on the beach - the destination for up to two million people on a hot summer day.