Items are sorted into crates to be sent to retail stores from the Argos Distribution Center in BurtonuponTrent, England, on Nov 27. Retail stores ramped up their operations to coincide with two sales events, Black Friday and Cyber Monday. Paul Ellis / AFP

Shopping through mobile devices soared on Cyber Monday, accounting for more than a quarter of the $3 billion in sales, but many online retailers struggled to get visitors using smartphones and tablets to shop as much as those using desktops.

Bigger-than-expected discounts and strong demand for electronics and toys made Cyber Monday the biggest day for online sales in the United States, according to the Adobe Digital Index report.

The report was based on data from 200 million visits to 4,500 retail websites on what is traditionally the busiest day of the year for Internet shopping.

Shopping using mobile devices accounted for 26 percent of total online sales, up from 19 percent a year earlier, Adobe's report said.

IBM's Watson Trend report said mobile traffic accounted for 47.9 percent of all online traffic, compared with 41.2 percent last year.

Top-selling items included Lego's Star Wars collection, the Barbie Dream House and Samsung 4K television sets.

Star Wars toys were among the items most frequently out of stock, Adobe said.

The average order value from online shoppers was $123.43, down 0.6 percent from last year, IBM said, mainly due to a rise in mobile shoppers.