Americans clicked away on their computers and smartphones for deals on Cyber Monday, which was expected to be the biggest online shopping day in history.

Shoppers were expected to spend $1.5 billion Monday, up 20 percent from last year, according to research firm comScore.

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That would not only make it the biggest online shopping day of the year, but the biggest since comScore started tracking shoppers' online buying habits in 2001.

Online shopping was up 25.6 percent on Cyber Monday compared with the same time period a year ago, according to figures released Monday afternoon by IBM Benchmark, which tracks online sales.

Sales from mobile devices, which include tablets, rose 10.9 percent. The group does not track the dollar amount of sales.

The strong start to Cyber Monday, a term coined in 2005 by a shopping trade group that noticed people were doing a lot of shopping on their work computers on the Monday following Thanksgiving, comes after overall online sales rose significantly during the four-day holiday shopping weekend that began on Thanksgiving.

"Online's piece of the holiday pie is growing every day, and all the key dates are growing with it," said Forrester Research analyst Sucharita Mulpuru. "The Web is becoming a more significant part of the traditional brick-and-mortar holiday shopping season."