Wal-Mart Stores, Target, Toys R Us, Macy's, J.C. Penney, and Best Buy were among the brick-and-mortar chains that started their online deals early.
"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.