Ten men, seven women, a girl and a boy both aged 10 were hit when wild gunfire opened up at about 1:45 p.m as the parade marched along North Villere Street, according to police spokesman Garry Flot.
Two victims are undergoing surgery, Flot said in a statement.
The children were hurt slightly but are in good condition, he said.
It was unclear if the victims were marching or bystanders watching the parade.
Police superintendent Ronal Serpas told reporters that officers saw three suspects running away, with one about aged between 18 and 22. No arrests were made.
“It appears that these two or three people, for reasons unknown to us, started shooting at, towards or into the crowd,’’ Serpas said, adding that the incident was over in “just a couple of seconds’’.
Serpas said a witness reported hearing two different types of gunshot, which he said indicated two weapons were involved.
Mayor Mitch Landrieu called the shooting part of “the relentless drum beat of violence’’ on the streets of New Orleans.
“It’s a culture of violence that has enveloped the city for a long period of time,’’ Landrieu told a news conference outside University Hospital, where three victims were being treated for serious injuries.
Photographs of the aftermath in the Times-Picayune newspaper showed a man lying on his stomach beside a pool of blood, being helped by two bystanders.
Other photos showed a man in shorts sitting on a cobbled street, his calf bleeding and covered with a bandana.
Emergency medical responders took 11 people to Interim LSU Public Hospital in New Orleans, according to hospital spokesman Marvin McGraw.
Violent crime in New Orleans ranks above the national average in FBI surveys. A poll of city residents in 2010 found crime to be their greatest concern.
In February, four people were wounded in a shooting outside a nightclub in the city’s French Quarter as crowds gathered for Mardi Gras celebrations.