By Jessica Wohl and Alistair Barr, Reuters/ Chicago/San Francisco

While it remains to be seen if the gains over Thanksgiving Day and Black Friday can be sustained, the latest estimates show shoppers scooping up early special offers via their smartphones and tablets, the way they used to hunt for bargains in brick-and-mortar stores.

Online sales increased 17.4% on Thanksgiving and 20.7% on Black Friday, compared with 2011, according to IBM Smarter Commerce, a unit of International Business Machines Corp that analyses transactions from 500 US retailers.

Black Friday, the day after Thanksgiving, had traditionally been the kick-off to the holiday season for stores. This year, retailers such as Wal-Mart Stores Inc and Target Corp made their biggest push ever with special offers during the holiday itself.

“The Thanksgiving creep revitalised the thrill for people,” said Wendy Liebmann, chief executive of WSL Strategic Retail. “It got people excited to go out. But it pulled a lot of sales forward.”

Visits to stores rose 3.5% on Friday, down from a 4.7% increase last year, ShopperTrak estimated.

Sales actually fell an estimated 1.8%, the company, which analyses store traffic, said. But if Thursday sales were added in, ShopperTrak estimates that ‘Black Friday’ sales would have been up a little less than 1% from a year earlier.

“It’s not a superstar Black Friday, but it also wasn’t an underperform Black Friday,” said Ed Marcheselli, chief marketing officer of ShopperTrak.

Cyber Monday, which follows the long holiday weekend, has been the biggest day for online shopping in recent years, as workers return to their office computers.

Now, armed with mobile devices, particularly Apple Inc’s iPad and iPhone, shoppers are no longer waiting. They are also using the devices to track down the lowest prices.

The average order value on Black Friday declined by 4.7% to $181.22, and the average number of items per order dropped 12% to 5.6, according to IBM.

Client sales on Black Friday rose 23% from last year, while Thanksgiving sales rose 32% year over year, estimated Mercent, which helps merchants sell more on websites including Amazon.com, ebay.com and Google Inc’s online shopping program.

“What we don’t know is whether this is a zero-sum game or whether there is some benefit to retailers by broadening the holiday shopping window,” Best said. “There is risk that real growth in retail for the entire holiday may be overestimated based on these early numbers.”

E-commerce software firm ChannelAdvisor said clients’ same-store sales on eBay’s online marketplace rose 27% on Thanksgiving and 31% on Black Friday, compared with a year earlier.

Last year’s year-over-year growth was about 15% for eBay, ChannelAdvisor said.

“The online players are definitely participating more on Thanksgiving and Black Friday, so Cyber Monday is being shared with other holidays now,” said Scot Wingo, chief executive ChannelAdvisor.

EBay usually grows more slowly early in the holiday season and then ramps up later as retailers run out of hot products and shoppers turn to eBay sellers for those items.

Its strong start this year is driven partly by the company’s success in attracting mobile shoppers through shopping apps on smart phones and tablets, Wingo said.

EBay said on Saturday that the volume of mobile transactions on its online marketplace jumped 153% on Black Friday, compared with a year earlier. PayPal, the payments division of eBay, saw Black Friday volume almost triple, year over year.

More broadly, purchases from mobile devices accounted for 16.3% of online sales on Black Friday, up from 9.8% in 2011 and 3.2% in 2010, according to IBM data. Apple Inc’s iPads and iPhones led the charge, with owners using the devices to browse and buy on retailer websites.

Overall, the percentage of sales from online shopping is expected to rise. But while retailers bring in more revenue from online shopping, they may also face some higher costs in terms of shipping a larger number of packages to more shoppers.

Among tablets, iPads were the clear leader, generating 88.3% of traffic to retailers’ sites on Friday, followed by Barnes & Noble Inc’s Nook with 3.1%, Amazon.com Inc’s Kindle with 2.4% and the Samsung Galaxy with 1.8%, IBM said.

At the mall, stores continued to use discounts to lure shoppers on Saturday, with Aeropostale Inc discounting items as much as 70% after a storewide 60% discount on Friday.

Rival American Eagle Outfitters Inc continued its two-day sale at 40% off, and Gap Inc’s namesake chain was offering 60% discounts for the entire weekend. “The discounts were reasonable but didn’t take your breath away,” said Liebmann. “Retailers are being cautious.”

The Garden State Plaza Mall in Paramus, New Jersey, appeared to be crowded on Saturday, although much quieter than on Black Friday, clerks at several stores said.

Vanessa Crenshaw, a 45-year-old accountant shopping at JC Penney, said that if stores pulled back on discounts, she would go elsewhere. “You can always find a deal, someone will have a deal,” she said.