NEW YORK - Retailers are hoping last week’s strong start to the holiday shopping season at malls and stores translates into a busy buying season online, which officially begins today.

The Monday after Thanksgiving, tagged “Cyber Monday” by the National Retail Federation, marks the first big online shopping surge for many merchants as consumers return to their work computers.

A number of retailers are hosting one-day sales or special offers for the occasion. Internet research firm comScore Inc. estimated online sales may exceed $700 million online today.

As for brick-and-mortar outlets, ShopperTrak RCT Corp., which tracks total sales at more than 50,000 retail outlets, reported late Sunday that sales on Friday and Saturday combined rose 7.2 percent to $16.4 billion from the same two-day period a year ago.

The biggest draw was electronics, benefiting chains like Best Buy Co. and discounters such as Wal-Mart Stores Inc. and Target Corp. Popular-priced department stores including J.C. Penney Co. and Kohl’s Corp. drew in crowds with good deals.

Toy stores like Toys “R” Us Inc. fared well too. Still, apparel sales appeared to be mixed at mall-based clothing stores, though a cold weather snap helped spur sales of outerwear and other winter-related items.

“This was a really good start,” said Bill Martin, co-founder of ShopperTrak. “There seemed to be a lot of pent-up demand.”

Now the attention moves online. Toys “R” Us Inc. will hold a one-day online sale and rival eToys.com will launch a two-day sale. Wal-Mart Stores Inc. will begin five days of online-only sales.

Online jeweler Blue Nile Inc. will give customers 20 percent off purchases paid through PayPal, eBay Inc.’s electronic payment division. Target Corp., Circuit City Stores Inc., Sears Holdings Corp., Crate & Barrel, the Discovery Store and Overstock.com Inc. are among dozens of retailers offering free shipping that day.

Nearly one-third of retailers are also having special one-day sales for Cyber Monday. Seventy-two percent plan some kind of promotion, according to NRF’s annual survey.

In fact, the number of retailers hosting online deals on the Monday after Thanksgiving has surged to 72 percent of those polled from 42 percent just two years ago.

Despite a decent showing, many shoppers interviewed said they planned to curb their spending. John Muller, of Clifton, N.J., who was standing outside Macy’s Herald Square in Manhattan on Sunday, said he plans to spend only about $500 this year, half as much as a year ago, because of higher expenses and worries about the economy.

This year, “we are mostly buying for the kids,” said Muller, who has two children, ages 3 and 7.