NEW YORK — The nation’s retailers had a robust start to the holiday shopping season, according to results announced Saturday by a national research group that tracks sales at retail outlets across the country.

According to ShopperTrak RCT Corp., which tracks sales at more than 50,000 retail outlets, total sales rose 8.3 percent to about $10.3 billion on Friday, the day after Thanksgiving, compared with $9.5 billion on the same day a year ago. ShopperTrak had expected an increase of no more than 4 percent to 5 percent.

"This is a really strong number. ... You can’t have a good season unless it starts well," said Bill Martin, co-founder of ShopperTrak, citing strength across all regions. "It’s very encouraging. When you look at September and October, shoppers weren’t in the stores."

In a separate statement released Saturday, J.C. Penney Co. reported "strong performance across all merchandise categories," including fine jewelry, outerwear, and young men’s and children’s assortments.

But the department store chain cautioned, "while we are encouraged by our strong start, it is still early in the holiday season, and we are mindful of the headwinds consumers are facing."

J.C. Penney, Wal-Mart Stores Inc. and other major retailers are expected to report same-store results for November on Dec. 6. Same-store sales are those at stores opened at least a year and are considered a key indicator of a retailer’s strength.

The upbeat reports were encouraging since merchants have been struggling with anemic sales in recent months, as shoppers, particularly in the middle and lower-income brackets, were becoming more frugal amid higher gas and food prices and an escalating credit crunch.

While Black Friday — so named because it was traditionally when the surge of shopping made stores profitable — starts holiday shopping, it is not considered a bellwether for the season.

However, merchants see Black Friday as setting an important tone to the overall season.

Last year, retailers had a good start during the Thanksgiving weekend, but many stores struggled in December, and a shopping surge just before and after Christmas wasn’t enough to make up for lost sales.

This year, the Washington-based National Retail Federation predicted that total holiday sales would be up 4 percent for the combined November and December period, the slowest growth since a 1.3 percent rise in 2002. Holiday sales rose 4.6 percent in 2006 and growth has averaged 4.8 percent over the last decade.