UPDATE 3-Cyber Monday spending rises 15 percent - comScore

(Adds ShopperTrak data, updates shares)

SAN FRANCISCO, Dec 3 (Reuters) - Online retail spending rose 15 percent from a year earlier on the all-important Monday after Thanksgiving, sounding a positive note in an otherwise bleak holiday season.

Still, the data from tracking firm comScore Inc (SCOR.O) also found e-commerce spending for the holiday season to date was off 2 percent.

Stifel Nicolaus analyst Scott Devitt said he was "pleasantly surprised" to see the 15 percent jump on "Cyber Monday." "The comScore data are certainly a psychologically relevant positive data point," he said.

Shares of Internet retailers rose in morning trading before falling back as the overall market turned negative and then rebounding once again in the afternoon.

Pent-up demand, combined with special promotions and door-buster deals, sent consumers shopping online and in brick-and-mortar stores over the Thanksgiving holiday weekend this year after months of penny-pinching.

And on Cyber Monday, when consumers log on -- often from work -- to seek better deals than they saw on visits to physical stores, online spending reached $846 million, up 15 percent from a year earlier, comScore found.

That made this Monday the second-heaviest online spending day on record, behind the $881 million spent on Dec. 10, 2007, comScore said.

But most retail experts doubt that the momentum seen over the holiday weekend will continue. This year's holiday season is expected to be one of the bleakest in years, as rising unemployment and tighter lending standards strain consumers' buying power.

At U.S. brick-and-mortar stores, sales rose a mere 0.9 percent during the three-day holiday weekend beginning Friday, said ShopperTrak on Wednesday.

And store traffic was down 19 percent in the same period, marking the largest three-day drop since the company began tracking that data. ShopperTrak monitors more than 50,000 retail locations.

Online retailer Amazon.com Inc (AMZN.O) and auctioneer eBay Inc (EBAY.O) have both warned that the holiday season would be challenging, and experts have voiced concerns that deep discounts from traditional brick-and-mortar chains could take sales from their online rivals.

Hitwise, an Internet tracking company, found that traffic to the top 500 retail websites was down 1 percent on Cyber Monday this year. But traffic at Amazon -- the most-visited online retailer on Cyber Monday -- rose 21 percent, while WalMart Stores Inc's (WMT.N) Walmart.com, which ranked second, rose 6 percent, according to the company.

Figures from Nielsen Online were better. That research service found a 10 percent traffic increase on Cyber Monday, with eBay, Amazon and Walmart.com taking the top spots.

Although Cyber Monday is rarely the heaviest online shopping day -- that tends to occur later in December, closer to Christmas -- comScore said the solid online spending showing for the first Monday of the traditional holiday shopping season was an encouraging sign for retailers.

"Consumers are clearly responding positively to retailers' aggressive online discounts," Chairman Gian Fulgoni said in a statement.