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.

IBM Benchmark reported Monday afternoon that online shopping was up 25.6 percent compared with the same time period a year ago.

Sales from mobile devices, which include tablets, rose 10.9 percent. The group does not track 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.

“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.”

It’s the latest sign that Americans are becoming addicted to the convenience of the Web. With the growth in smartphones and tablet computers, shoppers can buy what they want, whenever they want, wherever they want.

As a result, retailers have ramped up the deals they’re offering on their websites during the holiday shopping season, a time when stores can make up to 40 percent of their annual revenue.

Delisa O’Brien, 24, took advantage of some of the deals on Monday. O’Brien, who said she would rather shop online than deal with the crowds in stores, bought an HP Notebook for $399 on Hewlett Packard’s website for her mother. The company threw in a free Nook e-book reader with her purchase.

“When it comes to Black Friday, I’m a tiny, 5’1″ woman and the thought of having to push and shove my way through hoards of people just to get cheap merchandise is kind of a nightmare to me,” said O’Brien, a Brooklyn, N.Y. resident. “My mom gets a new laptop, I get an e-reader, and all without spending too much money. … Everybody wins.”

How well retailers fare on Cyber Monday will offer insight into Americans’ evolving shopping habits during the holiday shopping season.

Online sales on Thanksgiving Day, traditionally not a popular day for online shopping, rose 32 percent over last year to $633 million, according to comScore. And online sales on Black Friday were up 26 percent from the same day last year, to $1.042 billion.

For the holiday season-to-date, comScore found that $13.7 billion has been spent online, 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.