Millions of Americans turned out as the holiday buying season started earlier this year on Thanksgiving Day. But there's still a lot of shopping left.

While figures aren't yet available on how many people shopped on the first two days of the holiday shopping season, crowds came early and often as more than a dozen major U.S. retailers stayed open for 24 hours or more on Thanksgiving through the day after known as Black Friday.

But overall, The National Retail Federation predicts that 140 million shoppers planned to shop during the four-day holiday weekend that begins on Thanksgiving Day. And of those, about 23 per cent, or 33 million shoppers, planned to do so on the actual holiday.

So the challenge for retailers is to keep the momentum going through the weekend -- and beyond. In years past, stores have had a robust start to the season by offering deep discounts only to see crowds disappear until the final days before Christmas when the big bargains pop up again.

Overall, the retail trade group expects retail sales to be up 3.9 per cent to $602 billion during the last two months of the year. That's higher than last year's 3.5 per cent growth, but below the 6 per cent pace seen before the recession. And retail experts said it's going to be difficult for stores to get shoppers to keep coming back into stores without bargains.

"Can stores continue the momentum after a promotional November?" said Laura Gurski,a partner and global leader of the consumer product & retail practice at A.T. Kearney, a global management consultancy. "How do you top it in December?"

Despite that there is a lot of the holiday shopping season left, this year may cement the transformation of the start of the holiday shopping season into a two-day affair.

Black Friday had been the official start of the shopping season between Thanksgiving and Christmas. It was originally named Black Friday because it was when retailers turned a profit, or moved out of the red and into the black. Retailers opened early and offered deep discounts.

But in the past few years, store chains have been opening on Thanksgiving.

This year, several welcomed shoppers for the first time on Thanksgiving night, while Gap Inc., which owns Banana Republic, Gap and Old Navy, opened half its stores earlier on the holiday.

Wal-Mart stores, most of which stay open 24 hours, has for the past several years offered doorbusters that had been reserved for Black Friday. And Kmart planned to stay open 41 hours starting at 6 a.m. on Thanksgiving.

That has led some to question how much further Black Friday will creep into Thanksgiving, which along with Christmas is one of only two days a year that most stores are closed.

"Black Friday is now Gray Friday," said Craig Johnson, president of Customer Growth Partners, a retail consultancy.

The earlier openings have met with some resistance.

Workers' rights groups and some shoppers had planned protests on Thanksgiving and Black Friday to decry the way some store employees were forced to miss holiday meals at home. But as of Thursday afternoon, there were no reports of widespread protests.

Judy Espey ducked out of a Thanksgiving family dinner to buy a 50-inch flat-screen TV at Wal-Mart near Clifton Park, N.Y., for $288. But "I don't really dig the Thanksgiving night thing," she confessed. "I feel bad for the workers."

Vinnie Gopalakrishnan pledged not to hit the stores after seeing TV footage of people shopping on Thanksgiving. But he flip-flopped after his cousin told him about a deal on a big-screen TV.

"I said, 'I'm not going to do it. Those people are crazy,"' Gopalakrishnan said before heading to a Wal-Mart on Friday.

It's unclear whether the early openings will lead shoppers to spend more over the two days or simply spread sales out.

Last year, sales on Thanksgiving rose 55 per cent from the previous year to $810 million, as more stores opened on the holiday, according to research firm ShopperTrak. But sales dropped 1.8 per cent to $11.2 billion on Black Friday, though it still was the biggest shopping day last year.

Store sales numbers won't be available until Saturday. But IBM Benchmark, which tracks e-commerce for 800 retailers, said online sales on Thanksgiving were up 19.7 per cent from last year. Online sales on Black Friday rose 9 per cent, based on preliminary data.

There are signs that stores fared well, too.

Wal-Mart, the world's largest retailer, started its holiday sales events at 6 p.m. on Thanksgiving, two hours earlier than last year. Wal-Mart said customers bought at least 2.8 million towels, 2 million TVs, 1.4 million tablets, 300,000 bicycles and 1.9 million dolls.

Terry Lundgren, Macy's CEO, said the 15,000 people who showed up for the opening of the flagship store was the most ever, up from 11,000 last year. "Clearly people are in the shopping mood," he said.

Of course, not every retailer saw robust crowds.

At Woodland Hills mall in Tulsa, Okla., the owner of Bags and Bangle complained that he had to stay open from 8 p.m. on Thanksgiving to 10 p.m. on Friday. Suhail Zaidi, who was required by the mall to keep his booth open, said Thanksgiving was somewhat busy, but business had died down by 3 a.m. On Friday morning, he said he had seen only about 20 customers.

"We ruined the holiday," Zaidi said. "Black Friday is a good shopping day, but opening up on Thanksgiving is ridiculous."

AP writers Chris Carola in Clifton Park, N.Y., and Kristi Easton in Tulsa, Okla.; AP Retail Writer Mae Anderson in New York; and AP Business Writer Sarah Sell in Portland, Ore., contributed to this report.