With the holiday shopping season kicking into high gear, online retailers are off to a healthy start, with sales on the day after Thanksgiving up 11 percent from last year.

On Black Friday, the unofficial kick-off of the holiday shopping season, consumers spent $595 million online, compared to $534 million the same day last year, according to online metrics firm comScore.

"Black Friday, better known as a shopping bonanza in brick-and-mortar retail stores, is increasingly becoming one of the landmark days in the online holiday shopping world," comScore Chairman Gian Fulgoni said in a statement.

Fulgoni noted that the spike in sales could be partially attributed to the heavy discounts many retailers were offering Thanksgiving weekend as they court shoppers in what continues to be a sluggish economy. He said that the first workday after the holiday -- known as Cyber Monday -- and the early weeks of December would be the real test for the e-commerce season.

"That said, this is a very encouraging start," Fulgoni added.

Online shoppers also came out in force on Thanksgiving Day, spending $318 million, a 10 percent increase over 2008.

From the beginning of November through Black Friday, comScore has tallied e-commerce sales of $10.6 billion, 3 percent ahead of the same period last year.

Both comScore and another Internet data provider, Experian Hitwise, reported that Amazon (NASDAQ: AMZN) was the most-visited retail site on the Web, followed by Walmart.com (NYSE: WMT).

"Much attention has focused on Amazon and Walmart this season, and both retailers performed particularly well online on Black Friday in terms of attracting visitors," Fulgoni said. "We will be watching closely to see how these retailers perform during these next critical weeks of the season."

Online retail consulting firm Mercent also reported a healthy early start to the holiday shopping season, noting that consumers are responding to the heavy promotions e-commerce sites have been offering.

Mercent CEO Eric Best said that e-commerce spending is showing signs of "a measured recovery" for the retail sector.

Sales figures for Cyber Monday, which is generally one of the busiest e-commerce days of the year, will be released Tuesday afternoon.