At least four other people, including a woman eight months pregnant, were taken to hospitals for observation or minor injuries, and the store in Valley Stream on Long Island was closed for several hours. It reopened shortly after 1 p.m.
Wal-Mart Stores Inc. in Bentonville, Ark., called the incident a "tragic situation" and said the employee came from a temporary agency and was doing maintenance work at the store.
"The safety and security of our customers and associates is our top priority," said Wal-Mart representative Dan Fogleman. "Our thoughts and prayers are with them and their families at this difficult time. At this point, facts are still being assembled and we are working closely with the Nassau County Police as they investigate what occurred."
Nassau police said about 2,000 people were gathered outside the Wal-Mart doors at the mall about 20 miles east of Manhattan. The impatient crowd knocked the man to the ground as he opened the doors, leaving a metal portion of the frame crumpled like an accordion.
Nassau police spokesman Lt. Michael Fleming, who described the scene as "utter chaos," said, "This crowd was out of control."
Dozens of store employees trying to fight their way out to help the man were also getting trampled by the crowd, Fleming said. Witnesses said that even as the worker lay on the ground, shoppers streamed into the store, stepping over him.
Kimberly Cribbs, who witnessed the stampede, said shoppers were acting like "savages."
"When they were saying they had to leave, that an employee got killed, people were yelling 'I've been on line since yesterday morning,"' she said. "They kept shopping."
The 34-year-old man was taken to a hospital, where he was pronounced dead at about 6 a.m., police said. The exact cause of death has not been determined, and the man's name was not released pending notification of family.
A 28-year-old pregnant woman was taken to a hospital, where she and the baby were reported to be OK, said police Sgt. Anthony Repalone. At least three other people were taken to hospitals with minor injuries.
Police said criminal charges were possible in the case, but Fleming said it would be difficult to identify individual shoppers. Authorities were reviewing surveillance video.
The industry's largest retail group said the incident was rare.
"We are not aware of any other circumstances where a retail employee has died working on the day after Thanksgiving," said Ellen Davis, a spokeswoman at National Retail Federation.
Shoppers around the country line up early outside stores on the day after Thanksgiving in the annual bargain-hunting ritual known as Black Friday. It got that name because it has historically been the day when stores broke into profitability for the full year.
Many stores open early and stay open late. The Valley Stream Wal-Mart usually opens at 9 a.m.
Items on sale at the Wal-Mart store included a $798 Samsung 50-inch Plasma HDTV, a Bissel Compact Upright Vacuum for $28, a Samsung 10.2 megapixel digital camera for $69 and DVDs such as "The Incredible Hulk" for $9.
The store was crowded Friday afternoon after the store reopened but shoppers were more subdued. Fleming said the store didn't have enough security on hand to handle the early morning crowds. It wasn't immediately clear whether additional security was brought in when the store reopened.
AP retail writers Anne D'Innocenzio and Mae Anderson and contributed to this report.