The iPad 2 flew off store shelves in its opening weekend.

Apple may have sold anywhere from 400,000 to 500,000 units of the iPad 2 during the tablet's launch weekend, exceeding the company's expectations, according to Piper Jaffray analyst Gene Munster.

Those numbers compared with 300,000 of the original iPad sold during its debut weekend last April.

But the new numbers are even stronger when viewed as a one-day sales total.

Many retailers ran out of stock by late Friday, only hours after the iPad 2 launched, and were unable to replenish those supplies over the weekend, said Munster in a research note released Sunday night and sent to CNET. In checks of various retail chains throughout the weekend, including the Apple Store, Best Buy, and Target, Munster's team found the iPad 2 was virtually sold out everywhere.

Apple spokeswoman Trudy Muller issued the following statement today: "Demand for the next generation iPad 2 has been amazing. We are working hard to get iPad 2 into the hands of every customer who wants one as quickly as possible."

Based on interviews with 236 people in New York and Minneapolis who were waiting in line Friday for the new tablet, Munster believes that around 70 percent of iPad 2 buyers were new to the iPad, a change from typical launch days when most buyers are upgrading. That number compares with last year's release of the iPhone 4 when only 23 percent of the launch day buyers were new to the phone.

The number of new buyers is also crucial to Apple holding onto its dominance of the tablet market as the iPad is joined by an onslaught of competing devices.

"We believe this shows Apple is expanding its base of iPad users, which is critical to maintaining its early lead in the growing tablet market," wrote Munster. "As the user base grows, Apple's lead widens, and the company has a proven track record of building unmatched brand loyalty, which we believe will be a potent combination as the table market evolves."

Drilling down further, the analyst found that almost half of the customers surveyed opted for a 3G model. Coming out of the store, 41 percent of those questioned bought the 32GB version, while 30 percent went for the 64GB edition. Those numbers were higher than the percentage of original iPad buyers who chose those sizes during last year's launch weekend.

Based on strong demand both online and at retail outlets, Munster believes his estimate of 5.5 million new iPads sold for the first quarter, which ends in just a couple of weeks, may prove conservative.

Update, 8:00 am PT: Added statement from Apple.