ComScore Inc, an analytics firm whose data is closely watched by Wall Street, said U.S. online sales rose a total of 17.3 percent on Thanksgiving and Black Friday, the unofficial kick-off to the holiday spending season.

The National Retail Federation estimated on Sunday that retail sales over the full Thanksgiving weekend will reach $57.4 billion, with the average shopper spending $407.02, down from $423.55, because of lower prices.

(Reporting by Phil Wahba in New York; Editing by Chris Reese)