NEW YORK • Shoppers seem to be just as enthusiastic about shopping on their computers and smartphones on Cyber Monday as they were about finding deals over the weekend.

Online sales on Cyber Monday, which was started in 2005 by a retail trade group to encourage Americans to shop online on the Monday after Thanksgiving, were up midafternoon by 15 percent from a year ago, according to data from IBM Benchmark. Meanwhile, sales from mobile devices were up 7.4 percent. The group did not give dollar amounts.

The Cyber Monday numbers point to Americans' growing comfort with using their personal computers, tablets and smartphones to shop. Over the last few years, big chains such as Wal-Mart Stores Inc., the world's largest retailer, have been offering more and better incentives such as hourly deals and free shipping to capitalize on that trend. It's important for retailers to make a good showing during the holiday shopping season because that's when many of them make up to 40 percent of their annual revenue.

On Monday, Amazon.com offered its bigger, more expensive Kindle DX for $259, or $120 off the regular price. The Express clothing chain was giving 30 percent off and free shipping on all online orders. And Wal-Mart, which has been calling the holiday "Cyber Week" in ads, was offering an LG 47-inch LED TV for $879, or $320 off the regular price.

"Cyber Monday is far more exciting to me than Black Friday," says Jamie Minoso, 40, an English teacher from Alabama. "I do not enjoy the traffic and chaos involved in shopping at a mall."

To be sure, the strong start for Cyber Monday follows an even stronger kickoff to the holiday shopping season over the weekend. Americans shopped in record numbers, driven by earlier store openings and a push by retailers for online sales.

A record 226 million shoppers visited stores and websites during the four-day holiday weekend starting on Thanksgiving Day, up from 212 million last year, according to the National Retail Federation. And sales on Black Friday, the day after Thanksgiving, rose 7 percent to $11.4 billion, the largest amount ever spent, according to ShopperTrak, which gathers stores' data.

Online sales were strong even over the weekend. Thirty-eight percent of all purchases were made online this year, up from 31-32 percent last year, says Sherif Mityas, partner in the retail practice of A.T. Kearney, who believes the increase was due to heavy promotions.

Barneys, for instance, offered 40 percent off on its website on Thanksgiving Day, a day before it began its sales in stores. And Barnes & Noble offered 40 percent to 75 percent off online products, discounts that weren't available in stores.

"Retailers are doing a good job of creating more excitement online in ways they can't do in store," Mityas says. "They're creating that excitement of, 'I've got to get that special deal,' that is really spurring traffic."

It won't be clear how well retailers will ultimately fare on Cyber Monday until today. But last year, sales on the day topped $1 billion for the first time, making it the heaviest day of online spending ever.