U.S. online holiday sales are up 12 percent to more than $17.5 billion till date this year, with more than half of that making use of free shipping, according to research firm comScore Inc.

Retailers have increasingly responded to this consumer demand, with market leaders Amazon.com Inc and Wal-Mart Stores Inc, for example, both offering free shipping on virtually all transactions this season, comScore chairman Gian Fulgoni said.

Shoppers spent more than $800 million in four days in the latest week, led by Cyber Monday, which became the heaviest online spending day on record at $1.03 billion.

We anticipated that the post-Cyber Monday period would experience a slight hangover after many of the retailers' most aggressive deals and promotions expired, but we can expect to see activity begin to pick up again next week as we get into the middle part of December, Fulgoni said.

Cyber Monday, the Monday after the U.S. Thanksgiving holiday, marks the day when people returning to work after the break browse for gifts on office computers. This year, Cyber Monday fell on November 29.

ComScore's figures takes into account spending from the beginning of November.

(Reporting by Nivedita Bhattacharjee in Bangalore; Editing by Roshni Menon)