SAN FRANCISCO (Reuters) -

Holiday shoppers in the United States spent some $595 million online on Black Friday, up 11 percent from last year, with Amazon.com and Walmart.com the most visited sites, according to analytics firm comScore.

Visits to Amazon.com (AMZN.O) rose 28 percent, followed by the online unit of Wal-Mart Stores Inc (WMT.N), which grew 22 percent. The two companies have already engaged in a heated battle for market share this holiday, with each lowering prices on select merchandise.

Since November 1, some $10.57 billion has been spent online in the United States for the holiday season, representing a 3 percent gain, according to comScore (SCOR.O).

"This is a very encouraging start," said comScore Chairman Gian Fulgoni in a statement.

Earlier this week, the company said it expects total holiday e-commerce spending to rise 3 percent to $28.8 billion from $28 billion in the previous season when shoppers sharply curtailed spending amid the global financial meltdown.

The robust Web sales on Black Friday speak to retailers' attractive discounts online even on a weekend known for driving shoppers to brick-and-mortar stores, said Fulgoni.

"While this acceleration in spending suggests the online holiday season may be shaping up slightly more optimistically than anticipated, it may also reflect the heavy discounting and creative promotions being put forth by retailers that now encompass the use of social networks such as Facebook and Twitter," said Fulgoni.

Cyber Monday is the day considered the kick-off to the online spending season, when consumers return to work from the long Thanksgiving weekend and use their employers' fast Internet connections to purchase online.

(Reporting by Alexandria Sage, editing by Matthew Lewis)

Source: Black Friday We...