U.S. online sales are expected to hit $2 billion on "Cyber Monday," for the first time since the data firm comScore has been tracking such information.
A 20 percent increase is expected to come even after many consumers shopped earlier when retailers started offering their "Cyber Monday" deals on Sunday.
As of 12 p.m. EST, online sales were up 21.4 percent on "Cyber Monday," according to IBM Digital Analytics Benchmark.
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The growth is not just a boon for online retailer Amazon.com. Bricks-and- mortar chains that have successfully integrated their e-commerce and in-store businesses also stand to gain from the online growth.
"All of them are behind Amazon, but some are starting to catch up," said Edward Jones analyst Brian Yarbrough.
"Cyber Monday" is the busiest day of the year for online sales and follows the Thanksgiving weekend that kicks off the peak holiday shopping season.
To compete with each other and Amazon, many U.S. retailers began offering some of the season's best online deals, which were traditionally reserved for the Monday after Thanksgiving, several days ago.
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That helped overall online sales from Thanksgiving through Sunday rise 14.5 percent in 2013 over the same period last year, data from IBM showed.
"The earlier offering of deals online may weigh on Cyber Monday and, interestingly, some retailers are offering a worse deal on Cyber Monday than they had over the earlier period," said Nomura analyst Simeon Siegel.
U.S. shoppers spent almost 3 percent less than they did a year earlier during the Thanksgiving weekend, according to the National Retail Federation data. But online sales rose 17.3 percent on Thanksgiving and Black Friday, outpacing sales growth at brick-and-mortar stores, data from comScore Inc showed.
The S&P Retail Index was down 0.4 percent Monday afternoon.