Online shoppers in the U.S. set a record for one-day sales on Cyber Monday, racking up $733 million in purchases, comScore Networks said Tuesday.

The figure represents a 21 percent increase over the same day last year and the most money that U.S. shoppers have ever spent in a single day online, according to comScore.

Online shoppers have spent $10.74 billion in total so far this holiday season, which comScore measures from Nov. 1. That's 17 percent more than during the same period last year.

Cyber Monday is the first Monday after the U.S. Thanksgiving holiday, when people return to work and begin making online purchases, as opposed to brick-and-mortar store purchases they may have made on "Black Friday"-- the day after Thanksgiving.

Online stores have been struggling to keep up with the demand. On Monday, Internet performance analysis company Keynote Systems said 10 of the 30 major retail sites it monitors for its holiday retail index showed significant slowdowns.

The total number of online shoppers was up 38 percent this year, comScore said, although it noted an average decrease in spending of 12 percent.

Overstock.com and Circuit City's Web site were the two hottest sites compared to normal traffic, each experiencing jumps of over 130 percent. In contrast, Amazon.com visits were up only 50 percent, while Apple saw a 5 percent bump for its online store.

Thanksgiving Day and "Black Friday" also saw jumps in sales, with online consumers spending $272 million and $531 million on those days, respectively. Both topped sales for the same days in 2006, with respective increases of 29 and 22 percent.

ComScore did not provide an update on the most popular items for Cyber Monday, but as of early Wednesday morning, Nintendo's Wii games console and Microsoft's 30GB Zune media player were the two most popular items according to comparison shopping site PriceGrabber, positions they established over the holiday weekend.