Despite major retailers like Walmart (WMT), Target (TGT), and Best Buy (BBY) starting to offer many Cyber Monday deals on Sunday this year, the biggest online selling day of the year is already shaping up to be a blockbuster, generating record revenue.

Adobe, which tracks 80% of all online transactions at the 100 largest U.S. retailers, said on Monday that online sales had already hit $490 million in the morning, on their way to $3 billion for the day.

IBM, which also monitors e-commerce, is forecasting that online sales on Cyber Monday will rise 18% compared to last year, fueled by a boom in shopping on mobile devices. E-commerce rose 25.5% over the four-day Thanksgiving weekend, according to IBM.

MORE: Less Black Friday Madness as Customers Shift to Online Shopping

If the prediction comes true, it would continue what has turned out to be a historic weekend in retail: as Fortune reported on Sunday, a National Retail Federation survey found more Americans shopped online this weekend (103 million) than in stores (102 million for the the first time, a major turning point in shopper habits.)

A number of brick-and-mortar retailers like Target, J.C. Penney (JCP), and Kohl’s (KSS) all reported big jumps in online sales this weekend.

Of course, the online bonanza is fueled by deep discounts aimed at fending off Amazon.com (AMZN), something that could ding profit margins.

“While the topline is a definite plus—consumers are also out in full-force looking for great offers. That makes Cyber Monday a meek day for margins—and that is a challenge for retailers,” said Forrester Research analyst Sucharita Mulpuru.

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