The parades began around dawn, led by clarinetist Pete Fountain and his Half-Fast Marching Club. The Zulu krewe and the parade of Rex, King of Carnival, were to wind their way down stately St. Charles Avenue to the business district during the morning.
Parade-goers lined up despite the threat of inclement weather. Some families camped out overnight to stake out choice spots to view parades.
Freddie Zeigler, a National Weather Service meteorologist in Slidell, La., said there was an 80 percent chance of rain Tuesday. Fog blanketed the riverfront and business district in the early morning.
Still, no parades had been canceled and the Zulu parade began on time at 8 a.m.
Zulu had a pro football flavor. Baltimore Ravens' Super Bowl stars Jacobi Jones and Ed Reed, both Louisiana natives, were aboard with Warren Sapp, a retired NFL player.
Brian Gibson, 45, lounged in a folding chair while his wife and two daughters ran to the float in hope of beads, stuffed animals or the prized coconuts that Zulu members give away.
Mayor Mitch Landrieu led the Zulu parade on horseback in a black shirt and jeans.
Many revelers in the French Quarter had drinks in hand before sun-up. Some donned costumes, tutus, beads and boas.
"We'll be in the French Quarter all day," said Bobbie Meir of Gretna, La., with feathers in her hair and fingernails painted purple. "The sights today are jaw-dropping. It's a ton of fun and the best party in the world. Nobody does Mardi Gras like we do."
Costumes included spotted cows, bees, pirates and jesters. Many revelers were clad in costumes of purple, green and gold, the traditional colors of Mardi Gras.
Meir said her group of friends would be taking in the sights from a Bourbon Street balcony.
Throughout the day, street bands and costumed revelers will be parading through the city's old quarter as big floats and marching bands parade on the major thoroughfares.
Following the Zulu and Rex parades would be hundreds of trucks decorated by families and social groups.
Fountain's Half-Fast Marching Club stepped off from a staging point in the Garden District just after dawn. Others with colorful names such as the Jefferson City Buzzards would follow.
Fountain and his clubmates were clad in garish red suits and feathered hats as they got ready to march in the Garden District.
Fountain no longer walks the route, which will take him to the French Quarter, but rides a truck-towed trolley. As he boarded, parade-goers snapped photos with camera phones.
Fountain wasn't worried about the forecast.
"This is my life," he said, referring to his 63rd parade with the group he founded. "We're going to make it before it rains."
Bob Johnson sipped on a screwdriver as he prepared to march with Fountain. "This is a half-healthy drink," he joked.
Johnson has done parades on floats and has been with Fountain's street marchers for six years. "It's a whole different perspective than riding a float. You can get right up to people," he said.
In the French Quarter, where the revelry almost didn't stop overnight, crowds were expected to cruise down Bourbon Street, pleading for beads from revelers on balconies before heading to Canal Street for the parades.
Traditionally, the French Quarter is the scene of Mardi Gras' most ribald activities, while the streetcar line along St. Charles is given over to family groups who set for a day of barbecues and parade watching.
On the uptown parade routes, families carried on generations-old traditions of camping out overnight in tents to catch the earliest action. Many hoped to catch one of the colorfully decorated coconuts handed out by grass-skirt-clad riders on the Zulu floats.
Bryan Clark, 42, said it would take more than the threat of rain to break his family's tradition of camping near the place where the Zulu parade turns onto St. Charles Avenue. "We stay here rain sleet or snow," he said.
The New Orleans native said his parents hooked him on the Mardi Gras ritual and now he does it with his wife and children. "There is no other place in America you can do this," he said.
Parading was planned across south Louisiana and the Gulf Coast. In Louisiana's Cajun parishes, the tradition of the Courir du Mardi Gras was set to start after dawn, as groups of maskers on horseback would ride from community to community making merry.
Rain or shine, it was parade-goers last chance during the Carnival season, which ends with the stroke of midnight Tuesday night. After that, the solemn season of Lent replaces the revelry until Easter.
Frank Warford, of Riverdale, Ga., was holding an umbrella Monday as he walked Bourbon Street. He said he was ready to party through the rain.
"This is a party city. Everybody's partying and having fun, catching beads like crazy," said Warford, his neck draped in beads. "If it rains, put a hat on. It's as simple as that."
Scattered showers didn't keep revelers away Monday as thousands flocked to the French Quarter and along the Mississippi River for Lundi Gras festivities.
Many said they weren't going to let the weekend shooting on Bourbon Street wreck the party.
On Saturday night, four people were injured on the city's famed party street. One man has been arrested and police said they have issued an arrest warrant for a second suspect. A third person who is believed to have critical information about the shooting is also being sought, police said.