Cyber Monday kicked off the online holiday shopping season with the highest single-day ever of retail sales on the Web, a market research firm says.

Internet shoppers handed $608 million to retailers on the Monday following the Thanksgiving weekend, a 26% increase over the same day last year, comScore Networks said Wednesday. It was the first time single-day online sales broke $600 million.

"While it was certainly an historic achievement, the online holiday shopping season is just beginning to kick into high gear," comScore Chairman Gian Fulgoni said in a statement.

Indeed, Cyber Monday is unlikely to be the biggest online shopping day of the year, but it does mark the first major spike in online holiday spending. Market analysts expect online sales to peak closer to mid-December.

During the first 27 days of November, online retail spending reached $9.48 billion, a 24% increase over the same period in 2005, comScore said. The firm expects gift givers this holiday season to spend $24.3 billion in November and December, up 24% from last year.

Retailers have been luring shoppers to sites with significant discounts, Fulgoni said. "While online sales have certainly seen a strong start to the holiday season, it will be interesting to see if all of this discounting ends up cutting into retailers' profit margins."