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

Shoppers are expected to spend $1.5 billion on Cyber Monday, up 20 percent from last year, according to research firm comScore. 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. The group does not track dollar amount sales.

The strong start to Cyber Monday comes after overall online sales rose significantly during the four-day holiday shopping weekend that began on Thanksgiving.

It’s the latest sign that Americans are becoming addicted to the convenience of the Web. With the growth in high speed Internet access and the wide use of smartphones and tablets, shoppers can buy what they want, whenever they want, wherever they want. People are relying less on their work computers to shop than they did when Shop.org, the digital division of trade group The National Retail Federation, introduced the term “Cyber Monday.”

For the holiday season-to-date, comScore found that $13.7 billion has been spent online, marking a 16 percent increase over last year. The research firm predicts that online sales will surpass 10 percent of total retail spending this holiday season.