NEW YORK — Online holiday sales appear to be off to a strong start, with Cyber Monday promotions expected to give many kinds of retailers a significant boost over the next couple days, analysts said.

According to research firm comScore, online spending on Black Friday, the day after Thanksgiving, jumped 26 percent versus a year ago and followed a 32 percent increase on Thanksgiving itself.

Cyber Monday, coined in 2005 by a shopping trade group that noticed online sales spiked on the Monday following Thanksgiving, is the next key sales day for retailers.

This year's Cyber Monday is expected to be the biggest online shopping day of the year for the third year in a row. ComScore expects Americans to spend $1.5 billion, up 20 percent from last year on Cyber Monday, as retailers have ramped up their deals to get shoppers to click on their websites.

Janney Capital markets analyst Shawn Milne said sales from mobile devices like smartphones and tablets are a key driver of online sales this year and accounted for more than 15 percent of online Black Friday sales, with online auction and shopping site eBay and its PayPal payments service both reporting big jumps in mobile volumes.

Meanwhile, online retail giant Inc. also saw steep sales increases over the holiday weekend and is expected to make a big sales push on Cyber Monday, Milne said.

Benchmark analyst Daniel Kurnos also pointed to Amazon as a clear winner among the online retailers, noting that it was the most visited website and the website with the highest year-over-year growth rate in unique visitors among the top five retailers on Black Friday.

But he noted that other, less traditional, online companies could get a boost from a strong holiday shopping season such as online florists like and United Online Inc. He also pointed to Groupon Inc., noting that the fourth-quarter will be key for its Groupon Goods business, where revenue more than doubled in the third quarter and accounted for more than 25 percent of the company's total.

In premarket trading, Amazon shares edged up a penny to $239.89; eBay rose 32 cents to $49.33; and Groupon fell 9 cents, or 2.3 percent, to $3.86.