"Cyber Monday" online retail sales got off to a strong start, International Business Machines Corp's IBM Benchmark unit said.

Online sales as of midday Eastern time in the United States were up 20 percent versus the same period last year, according to IBM Benchmark.

Cyber Monday is traditionally the first day after Thanksgiving when employees return to offices -- and purchase items using their work computers.

In 2010, Cyber Monday sales topped $1 billion, making it the heaviest day of online spending ever, according to comScore Inc. This year, the firm expects Cyber Monday sales to exceed last year's tally.

"We expect most shopping to come from laptops and PCs during the day with mobile shopping activity picking up later," IBM Benchmark said in a statement.

Online sales have grown strongly in the first part of the crucial holiday shopping season, which starts around Thanksgiving and runs through Christmas.

ChannelAdvisor, a software company that helps retailers sell more online, said clients saw same-store sales via Amazon.com Inc jump 59 percent on Sunday, versus the same period last year.

ChannelAdvisor's Amazon gross merchandise value hit a record on Saturday, setting the stage for what is likely to be its highest-ever Cyber Monday today, the company added. (Reporting by Alistair Barr, editing by Gerald E. McCormick)