The online holiday shopping season got off to a solid start, according to data released on Tuesday by ComScore Inc.

U.S. online spending reached $9.7 billion in the first 20 days of the holiday season, which began in early November, ComScore said. That was up by at least 14 percent from the same period a year earlier, the company added.

Online retailers have entered the most important part of the year, when billions of dollars in sales are up for grabs.

In 2010, U.S. shoppers spent almost $22 billion online between early November and December 10, up 12 percent from the same period a year earlier, ComScore data show.

In the first three weeks of this November, same-store sales generated by ChannelAdvisor's online merchant clients jumped 28 percent from the same period a year earlier.

ChannelAdvisor helps merchants sell more on third-party marketplaces run by e-commerce giants including Amazon.com Inc and eBay Inc.

ChannelAdvisor merchants saw same-store sales jump 65 percent on Amazon.com in the first three weeks of November, versus the same period of 2010.

Merchants on eBay saw a 19 percent increase in same-store sales during the same period, ChannelAdvisor also said.

All channels firing, ChannelAdvisor Chief Executive Scot Wingo wrote via Twitter on Tuesday.

(Reporting by Alistair Barr, editing by Bernard Orr)