Four people were shot on the French Quarter's iconic Bourbon Street, sending people running as revellers partied Saturday night amid the countdown to Mardi Gras, police and bystanders said. But the party was back in full force hours later as crowds returned afterward.

Two males and two females were wounded just before 9:30 p.m. time, New Orleans police spokesman Frank B. Robertson said. He reported that one male was in critical condition and had undergone surgery, while the other three were in stable condition. He did not release their ages.

Robertson said detectives were working to identify a suspect and determine a motive. A police statement said the shooting occurred on the French Quarter street, but did not provide the exact location where the shots were fired. He said he had no additional information immediately.

"They're just piecing together what happened," he added.

Police have released a bystander's video of what they say is an argument involving some people in a Bourbon Street crowd celebrating ahead of Mardi Gras when shots ring out, followed by screams.

Police spokesman Frank B. Robertson says the shooting left four people wounded, including a male victim shot in the abdomen, thigh and pelvic area.

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Members of the Krewe of Endymion parade down Orleans Avenue in New Orleans on Saturday ahead of Mardi Gras. (Sean Gardner/Reuters)

The footage showed many partying in costumes before what police described as an argument between the most seriously injured male and two other men. Four rapid shots are heard, then screams.

Authorities have not said if they have made any arrests, saying they sought the public's help. 

The incident occurred as the streets were crawling with bar-hopping throngs taking in the last weekend before Fat Tuesday, the enormous party that engulfs New Orleans each year with parades, gaudy floats and merrymakers tossing trinkets and beads to the crowds.

Bourbon Street street is home to strip clubs, watering holes and second-floor balconies lined by people who throw beads to revellers below each Mardi Gras season. The street often gets so crowded that officers have to control the crowds on horseback.

Patrick Clay, 21, an LSU student, told The Times-Picayune he was standing on the corner of Bourbon Street when suddenly he saw a crowd running and people screaming that there was a shooting.

"Everyone immediately started running and the cops immediately started running toward where people were running from," Clay said. "I was with a group of about seven people and at that point we all just kind of grasped hands and made our way through the crowd as soon as possible."

Afterward, police moved in to investigate. Many revellers said they stayed hunkered down in bars and other establishments until police cleared them to move freely.

WWL-TV reported that police had obtained surveillance video from one of the establishments as part of the investigation. "We don't know what happened but they shut down the entire block for an hour," Peter Manabani, an employee at the Rat's Hole bar, told AP as loud music thumped in the background.

He said the block reopened shortly before midnight and his establishment was again thronged entering the early hours. Early Sunday there were no signs a shooting had occurred, as revellers had returned to party mode, packing the block anew amid a heavy police presence.

Many milled about, wearing beads, drinking and carousing.

"It's scary. We heard about the shooting in the cab ride down here and almost turned around but it's our first Mardi Gras and we wanted to be here," said Ashley Holleran, 19, of Allendale, N.J., visiting with a friend from New York.

'We were just locked in a bar and we weren't going to let this one incident wreck our party.'—Laura Gonzalez, 21, of Baytown, Texas

Laura Gonzalez, 21, of Baytown, Tex., said it was also her first Mardi Gras and she spent some time in the Fat Catz Bar nearby as police investigated the shooting.

She said the bar quickly locked its doors soon after the shooting and wouldn't let anyone in or out while police kept the crime scene clear of throngs. Asked if it was frightening, she responded: "Not really. We were just locked in a bar and we weren't going to let this one incident wreck our party."

Parades rolled all day Saturday but none on Bourbon Street because the streets are too narrow. One of the biggest Mardi Gras parades, the Krewe of Endymion, rolled down Canal Street and just skirted Bourbon Street a few hours before the shooting. Typically, once the parades end, partygoers head to the French Quarter.

The lifeblood tourism trade is vital to New Orleans and Mardis Gras is one of the city's signature events, along with Jazz Fest and major sporting events such as the recent Super Bowl.

Yet decades-old problems persist and New Orleans remains plagued by violent crime, including gun violence that soared after Hurricane Katrina clobbered the city in 2005.