Predictions of strong online sales growth for Cyber Monday were panning out, according to one early indicator Monday, with more consumers using their smartphones and tablets as shopping tools.

Overall online sales on Cyber Monday -- an event that often ranks among the biggest for online spending -- were up 17.5 percent by 6 p.m. compared to the same period last year, according to IBM Digital Analytics Benchmarks, a platform that tracks millions of transactions and data from about 800 retail sites nationwide.

"Online shopping is having a banner year," with Cyber Monday expected to lead the way, said Jay Henderson, strategy director for IBM. "The biggest shift we've been tracking is the shift to mobile devices."

Smartphones and tablets accounted for 29.4 percent of all online shopping traffic and 15.6 percent of all online sales as of 6 p.m. Monday, according to IBM. Consumers generally used their smartphones to browse and made more purchases with their tablets, he said.

Consumers using tablets averaged $129.82 per order and made 9.6 percent of all online sales. Smartphone purchases averaged $112.34 per order and accounted for 5.9 percent of all online spending Monday, IBM said.

An earlier survey by the National Retail Federation, a trade organization, estimated that more than 131 million shoppers were planning to go online for Cyber Monday, up from 129 million last year.

Locally, one executive said online business has been strong since Thanksgiving.

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"Cyber Monday started on Thursday," said Jeffrey Ehrlich, president of Fulfillment Plus Inc., a Holtsville company that packages and ships online orders for retailers. "A lot of online retailers had pre-Cyber Monday specials, trying to have consumers purchase online instead of going to the stores on Thursday and Black Friday."

Fulfillment Plus was on track to ship about 20 to 25 percent more orders than it did last year between Thanksgiving and Monday, Ehrlich said. Monday was a day of "controlled chaos," he said.

But more days of shopping promotions don't necessarily mean more sales, said Marshal Cohen, retail analyst for The NPD Group, a Port Washington market research firm.

From Thanksgiving through Sunday, 2 million more people than last year shopped online and in stores, the NRF estimated on Sunday. However, average spending per consumer dropped 3.9 percent from last year and estimated total spending fell 2.7 percent.