NEW YORK (CNNMoney.com) - Online auctioneer eBay was the clear winner among e-tailers who garnered the most traffic this week on the "official" start of the online holiday shopping season, also known as Cyber Monday.

Market research firm Nielsen NetRatings' Holiday eShopping Index saw 15 percent more traffic Monday than on Black Friday, or the day after Thanksgiving, which typically marks the kickoff to the traditional start of retail industry's holiday gift-buying season.

The report showed Cyber Monday traffic drew a combined total unique audience of 27.7 million from people who shopped online both from home and work.

Nielsen//NetRatings, which tracks traffic to more than 100 retail Web sites, said eBay (Research) had 11.7 million unique visitors, followed by Amazon (Research) with 5.6 million on Cyber Monday. Rounding out the top five Web sites were Walmart.com with 3 million unique visitors, Target.com with 2.2 million and Overstock.com with 1.9 million visitors.

Dell.com narrowly missed out on the top five, coming in sixth with 1.88 million.

"Cyber Monday brought a surge of traffic to many online retailers as shoppers returned to work following the holiday weekend ready to tackle their holiday gift lists," Nielsen/NetRatings senior retail analyst Heather Dougherty said in the report late Tuesday.

"Heavy promotional activity will entice shoppers to take advantage of the discounts and free shipping offers that are being offered online," she added.

Separately, comScore Networks reported that non-travel spending on Cyber Monday increased 26 percent to $485 million this year.

But despite the hype over Cyber Monday, the firm said it expects the highest online spending day of the season will come closer to Christmas.

"Black Monday was only the 12th highest online spending day of the 2004 holiday season. Last year, peak sales days actually occurred in mid-December as consumers scrambled to take advantage of late-season discounts and free-shipping offers," Gian Fulgoni, chairman and co-founder of comScore Networks, said in a report.

Over the Thanksgiving weekend, including last Thursday through Sunday combined, consumers spent more than $925 million online, an increase of 26 percent over the same period in 2004, the comScore report said.

ComScore forecasts that total holiday online sales, excluding travel, will grow 24 percent over last year to $19 billion during the key November-December shopping months.