High traffic disrupted Wal-Mart Stores Inc.'s (WMT) Web site for much of Friday, one of the year's busiest shopping days.

The troubles came a day after Amazon.com Inc.'s site had brief disruptions because of a Thanksgiving Day sale on Microsoft Corp.'s (MSFT) Xbox 360 video game machines.

For much of Friday morning, attempts to open Walmart.com resulted blank pages, delays or other problems. By early afternoon, visitors were simply told to come back later.

Walmart.com spokeswoman Amy Colella blamed a "higher than anticipated traffic surge."

Black Friday, the day after the Thanksgiving holiday, marks one of the year's busiest days for retailers and the official start of the holiday shopping season.

The site appeared to be back to normal mid-afternoon Friday, after frustrating countless potential shoppers.

"People who wanted to purchase one of those plasma TVs, ... they probably went to a competitor potentially," said Ben Rushlo, senior manager of competitive research at Keynote Systems Inc.

Keynote, which regularly monitors performance at leading Web sites, said its probes began detecting problems at about 4:30 a.m. EST. Throughout the morning, visitors still could access portions of the site but generally ran into difficulty before completing purchases.

For instance, searches that normally take a second or two were taking 30 or 40 seconds, Rushlo said, and attempts to log in or pay for purchases sometimes generated error messages.

Walmart.com is the 21st most popular site in the U.S., with 22.8 million unique visitors in September, according to comScore Media Metrix.

Rushlo said retailers get better each year bracing for the volumes, but they also make their sites increasingly complex, adding 360-degree views of products and other features. Nonetheless, with the exception of Wal-Mart, online retailers were generally performing well.

"There were a few glitches here and there, minor problems," Rushlo said.

He said Amazon.com's troubles were relatively small.

The site was disrupted for about 15 minutes, starting at about 2 p.m. EST Thursday, as the retailer was offering the Xbox 360 to the first 1,000 customers for $100, $200 below the regular retail price.

"We saw dramatically more traffic than what we anticipated," Amazon.com spokesman Craig Berman said Friday.

The Xbox sold out in 29 seconds, Berman said. Amazon also sold out of discounted Mongoose mountain bikes, Barbie dolls and Amazon Prime memberships with $100 gift certificates in about 15 minutes.

Roughly 137 million people are expected to hit the mall from Friday to Sunday, according to the National Retail Federation, making it one of the most lucrative weekends for stores.

Millions of people are also expected to shop online starting this weekend. The Monday after Thanksgiving, also known as "Cyber Monday," is expected to be one of the year's biggest online holiday shopping days, as people return to work and shop online using their office computers.