Online holiday shopping continues at a blistering pace.

For the week ending Dec. 16, more than $6.3 billion was spent on online shopping, according to digital analytics firm comScore. This figure is 15 percent more than last year.

Notable shopping days included Green Monday (Dec. 12) and Free Shipping Day (Dec. 16), during which sales for both of those days totaled more than $1 billion and were up 19 percent and 14 percent, respectively, compared to last year.

Altogether, online holiday shopping sales have totaled $30.9 billion for the first 46 days of the holiday shopping season, a 15 percent increase compared to last year.

 More than $1 billion in spending on Free Shipping Day put the exclamation point on what will almost certainly be the heaviest week of the online holiday shopping season, comScore chairman Gian Fulgoni said in a statement.

While next week may see another strong day or two at the beginning of the week, it's clear that we have now reached the crescendo for this season and that spending will begin to slow as we get closer to Christmas, leaving Cyber Monday as the top-ranked shopping day for the second year in a row, the statement continued.

Sales on this year's Cyber Monday totaled $1.25 billion, a 22 percent increase compared to last year.

Last month, comScore predicted that online shopping sales for the holiday season would total $37.6 billion. The firm defines the holiday season as Nov. 1 to  Dec. 31.

Separately, comScore notes that free shipping is on the rise. Fifty-six percent of online sales transactions during the week ending Dec. 16 involved free shipping, up 3.8 percent compared to 2010.