Crowds were building at stores in and around New York, Philadelphia, Chicago and Los Angeles by midday on Saturday. Some shoppers said they were drawn by the discounts, which have been deeper and lasted longer this year. Others said they felt more inclined to spend because of lower prices at the gas pumps.

Alexis Layne, a 53-year-old retired schoolteacher shopping at a mall in downtown Chicago, said she usually buys only "fun stuff like accessories" the weekend before Christmas but had made a few impulse buys like two Lacoste T-shirts for her husband because the deals were too good to pass up.

Ramesh Swamy, a retail analyst at Deloitte, said retailers were anticipating a strong weekend of sales. One of the malls he visited in the Los Angeles area was preparing overflow parking spaces, and he saw some retailers restocking shelves.

"I think folks are willing to open up their purse a little bit more this year," he said, pointing to lower gas prices and solid corporate profits as supporting factors. "It should be a pretty good day."

Retailers are banking on big sales this weekend to help make up for a spotty performance so far, including a disappointing "Black Friday," the day after Thanksgiving that typically ranks as the busiest shopping day of the year.

ShopperTrak, which surveys traffic at bricks-and-mortar stores, expects today - called "Super Saturday" by retailers because of its importance to sales - to surpass Black Friday as the top revenue-generating day of 2014.

According to the National Retail Federation, retail sales rose an unadjusted 3.2 percent in November, trending below its holiday forecast for 4.1 percent during November and December combined. Its chief economist said the forecast was still achievable, with a strong December to pick up the slack.

Aside from pockets of rain in Georgia and South Carolina and parts of the Pacific Northwest, weather conditions were generally good with temperatures cold enough in the East to spur sales of sweaters and other winter goods, said Evan Gold, a senior vice president at Planalytics, which advises companies on how weather conditions impact business.

"I think it should be a strong day," he said.

JC Penney's flagship store in Manhattan was starting to fill up by midday, after a slow start. Annette Rozzo was one of three shoppers waiting when the doors opened at 7 a.m. With a coupon for 60 percent off clothing, she purchased six sets of pajamas and said she was planning to pick up toys from the Disney movie "Frozen" for the kids. The deals were better than 2013, she said, and she would "definitely" spend more than last year.

Some analysts have warned that retailers are pushing sales at the expense of margins. Many launched deep discounts right after Halloween and have kept them all the way through. Price cuts for apparel have been especially steep.

Emma Clement, 37, said she felt she was still getting great deals even though she left the bulk of her holiday shopping to the final weekend as she was busy traveling for work.

"Have you seen some of the deals today? They are hard to beat," she said after seeing discounts of up to 70 percent on designer clothing at a Macy's in the Chicago area mall. "Most stores are offering over 50 percent off."

(reporting by Samantha Sunne in New York; Nandita Bose and Nathan Layne in Chicago; editing by Gunna Dickson)

By Nandita Bose and Samantha Sunne