By Karen Jacobs

ATLANTA (Reuters) - Buyers spent $733 million bargain-hunting on "Cyber Monday," up 21 percent from a year earlier, and some analysts expect that one-day tally to be topped as Christmas nears.

The biggest online traffic winners on the Monday after Thanksgiving included retailers Amazon.com Inc , Wal-Mart Stores and Best Buy Co , Internet research firm comScore Inc said on Wednesday.

Cyber Monday marked the first time that a single day of online spending broke the $700 million mark, comScore said.

The Monday after the four-day holiday weekend has been dubbed Cyber Monday because many consumers return to work and seek deals not found in bricks-and-mortar retailers using their employers' high-speed Internet connections.

ComScore said 60 percent of the money spent online on Monday came from work computers.

"It's clear that Cyber Monday is the biggest day of the 2007 holiday shopping season so far," said Ken Cassar, an analyst with Nielsen Online.

Nielsen data indicated 32.5 million people shopped online on Cyber Monday, up 10 percent from a year earlier and higher than the 28.8 million who shopped online on Black Friday, the day after Thanksgiving that kicks off the holiday season for offline retailers.

Cassar and others said the Cyber Monday figures would likely be surpassed in the weeks leading up to Christmas.

Though retail watchers expected online sales to spike on Cyber Monday, some say the phenomenon could be waning as retailers lure online shoppers earlier in the season.

Historically, online shopping picks up as the holiday season progresses.

"(Monday,) December 17 could be the heaviest day" of the online shopping season, based on typical trends, said Andrew Lipsman, a senior analyst with comScore.

He noted that while Cyber Monday sales spiked in 2006, the day wound up as just the 12th-heaviest online spending day once the holiday season was over.

"Spending continues to build over the next couple of weeks, and it usually peaks in December," Lipsman said.

BARGAIN HUNT

Deep discounts were a factor this year.

A survey by BizRate Research of 116 online retailers found that 72.2 percent planned a special promotion for Cyber Monday, up from 42.7 percent of online merchants two years ago.

"It does seem like the promotions get more aggressive each year," said Lipsman.

But those bargains could backfire later in the holiday season as rising food and fuel prices and a troubled housing market threaten to dampen consumer spending, Cassar cautioned.

ComScore said the number of Cyber Monday buyers rose 38 percent from a year ago, but average spending per buyer slipped 12 percent partly because of discounting.

"If people did shop heavily on Cyber Monday driven by promotions, are these sales that won't happen later in the season?" Cassar said.

"There is some risk that retailers may push sales early in the season at the expense of higher-margin sales later," he added.

Other highly trafficked sites on Cyber Monday included those belonging to Target Corp and computer maker Dell Inc , according to comScore.

Sanford Bernstein analyst Jeffrey Lindsay said he expects a strong fourth quarter for retailers Amazon and eBay Inc , as more consumers seek deals online.

"We believe that large numbers of consumers are already using the Internet to find bargains, save on time and gasoline by avoiding store visits, and get better selection and choice online," Lindsay said in a research note on Wednesday.

(Additional reporting by Justin Grant in New York; Editing by Brian Moss)
© Copyright 2007 Reuters.