Organizers earlier said they expected 2.4 million, which would have qualified as the world's biggest street party in the Guinness World Records.
"The Rio Carnival starts in earnest only when Bola takes to the streets," the daily O Globo quoted Pedro Martin, the bloco's president, as saying.
"Bola Preta is a Rio institution of which we are very proud," he added.
Black polka dots on white are the trademark style for Bola Preta (black ball) fans since -- according to city lore -- the bloco's name was inspired by a beautiful, curvy woman in a polka dot dress.
"I come every year. The more people the merrier," said Luiz Ornellas, a 25-year-old student dressed as a clown.
The street bash got under way a day after the legendary King Momo officially declared the start of the 2013 Rio Carnival after symbolically receiving a giant key to the city from Rio Mayor Eduardo Paes.
Carnival is the most popular festival in Brazil and is celebrated with equal gusto in most regions.
As is customary, Salvador, the heart of Afro-Brazilian culture, led the way Thursday night, with hundreds of thousands of people partying in the streets of the Bahia state capital.
This year, Brazilians are paying a special tribute to South Korea to mark the 50th anniversary of Korean immigration to the country.
Samba schools in Rio and Sao Paulo are honoring the contribution the 50,000-strong Korean community has made to this vibrant and racially diverse country of 194 million people.
In Salvador, South Korean pop star Psy performed to huge crowds alongside Brazilian stars Claudia Leitte, Gilberto Gil and Daniela Mercury.
The rapper wowed his audience with his "Gangnam Style" hit -- featuring his signature horse-riding dance -- that made Internet history last December by clocking more than one billion views on YouTube.
Sao Paulo, the country's most populous city and economic capital, kicked off its elaborate Carnival parades Friday, with a tribute to popular sambista Beth Carvalho, who is in hospital recovering from spinal surgery.
With colorful fantasy floats and elaborate costumes, other samba schools highlighted Sao Paulo's rich ethnic and racial diversity and celebrated the world's other major festivals, such as the Chinese New Year, the Venice Carnival, New Orleans Mardi Gras and Mexico's Day of the Dead.
On Rio's streets Saturday, Bola Preta fans strutted along in a variety of eccentric costumes.
Spotted in the crowd was a President Barack Obama figure waving a flag proclaiming "I am the Man," while another reveler was decked out as the late pop king Michael Jackson.
Other popular costumes included rabbits, bees, fairies, princesses, firefighters, policemen and superheroes.
The crowds were so dense that many people had to climb atop police cars.
In the northeastern city of Recife, an estimated two million Carnival partiers jammed the streets, singing and dancing, spurred along by copious beer drinking.
The joyful merrymaking came on the eve of Rio's eagerly-awaited parades of richly decorated floats and scantily-dressed beauty queens at the Sambodrome parade ground designed by the late Oscar Niemeyer.
Twelve elite samba schools are vying for the title of Carnival champion in a fiercely contested showdown watched with the same fervor as football matches in this soccer-mad nation.
While the Sambodrome parades are the highlight of the Rio Carnival, they attract mainly the wealthy and foreign tourists.
The blocos, by contrast, are open to all, drawing rich and poor from all city districts and neighboring towns.
Authorities said six million people, including more than 900,000 tourists, were expected to attend some 492 street parties during the five-day Rio extravaganza.
The Marvelous City, which will host next year's World Cup and the 2016 Summer Olympics, has mobilized 14,500 police officers to provide security.
For many Brazilians, the celebrations are a chance to forget the January 27 nightclub fire in the southern college town of Santa Maria that left 239 young people dead.
The tragedy prompted authorities to step up safety inspections at entertainment spots nationwide.
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