Online US shopping for Black Friday deals soared to $3 billion during a two-day period beginning Thursday, with tablets and cell phones as top must-have items, estimates showed. Online purchases reached $1.93 billion on Friday itself, the unofficial start of the retail sector's holiday season. That marked a 39 percent increase over 2012, according to software maker Adobe, which analyzed 400 million visits on some 2,000 American shopping websites. Holiday shopping traditionally accounts for 20 to 40 percent of an individual retailer's annual sales, according to the National Retail Federation. Early online Black Friday sales, which began on Thursday, the US Thanksgiving holiday, reached $1.06 billion, up 18 percent from last year, according to Adobe. Technology giant IBM also reported increased numbers in overall online sales as it looked at 800 merchant websites. IBM said Internet sales jumped 19.7 percent over last year on Thanksgiving and 19 percent on Black Friday, with orders averaging $135.27, a 2.2 percent increase compared to last year. Online shoppers may have been wise to avoid stores, with reports of fistfights, a stabbing and a shooting as people elbowed their way through crowded shopping floors to snatch heavily discounted items. Purchases of smartphones, tablets and other mobile devices accounted for 24.2 percent of online sales, according to Adobe. Tablets represented 15.6 percent of online sales while smartphones representing 8.6 percent. IBM also found that mobile devices accounted for 21.8 percent of sales. According to Adobe, of the $3 billion in total online sales over the two days, $417 million was done on iPads and $126 million was done on iPhones, while Android phones were used to buy $106 million in purchases and Android tablets accounted for $42 million. IBM said tablets were used for 14.4 percent of online sales, against 7.2 percent on smartphones. On average, tablet users each spent $132.75 and smartphone users spent $115.63. The company also found that iPad and iPhone users spent more, shelling out an average of $127.92, compared to $105.20 for users of Google's Android system. Adobe agreed that Apple users spent more than people using Android devices. IBM also said purchases made from Apple devices accounted for 18.1 percent of total online sales, against 3.5 percent for Android devices. IBM and Adobe did not examine in-store purchases. According to initial estimates of in-store sales released Saturday by retail research group ShopperTrak, Americans spent $12.3 billion during shopping over the two-day period, a 2.3 percent increase from the same timeframe in 2012. According to the firm, stores that opened on Thursday -- a new trend transforming the sacrosanct Thanksgiving holiday -- attracted more people than last year. Traffic in brick-and-mortar shops on Black Friday, however, fell 11.4 percent, with retail sales decreasing by 13.2 percent. "The Black Friday shopping experience is changing with more shoppers choosing to go out on Thanksgiving Day," said Bill Martin, ShopperTrak founder. "Consumers increasingly research products online before entering stores. When they arrive, customers know exactly what they want to buy," he said. An estimate of total Thanksgiving weekend sales -- including both online and in stores -- is expected Sunday. This season's holiday shopping season is a full six days and one shopping weekend shorter than last year's due to the late Thanksgiving season.