Apple has announced that customers have downloaded more than one million copies of Mac OS X Lion from the Mac App Store in the upgrade's first day of availability.

Apple boasted of the initial success of Lion, with wording that hinted it has raked in about $30 million (£18.5 million). "In just one day, over one million users bought and downloaded Mac OS X Lion," Apple said.

The phrase "bought and downloaded" could be interpreted in other ways, however. Because Mac owners can buy a single copy of Lion which can then be installed on all of their machines, it's possible that first day revenues were far less than the headline figure.

Apple also said that Lion was selling "faster than any other OS release in [the company's] history." A direct comparison to Lion's predecessor, Snow Leopard, is impossible: Apple did not release early sales figures for that operating system after its August 2009 debut.

In September 2009, retail research firm NPD Group estimated that Snow Leopard sold twice as many copies in its first two weeks than its predecessor, 2007's Leopard, and almost quadruple the number posted by Tiger, which shipped in 2005. Snow Leopard sold for less than Leopard or Tiger, one reason why that edition flew off the shelves, NPD Group analyst Stephen Baker said two years ago.

In October 2007, Apple claimed that it had sold or delivered 2 million copies of Mac OS X 10.5 Leopard in the first fours days. But the Leopard number included copies installed on new Macs sold during that stretch.

Lion's faster sales pace could stem from a larger number of Macs in use now than in 2009. Apple has sold 28.5 million Macs worldwide since Snow Leopard's launch. Its online-only distribution which doesn't require customers to drive to a physical shop or wait for an online order to be delivered, could also add to the swift uptake.

Apple plans to sell Lion on a USB flash drive from next month, and the company also sells Lion Server as an add-on/upgrade to Lion.

Not surprisingly, Lion has held the top spot in the Mac App Store's bestseller list since its launch. When checked, Lion Server was in the third position on the paid app list. By late Thursday, Lion had collected over 9,500 customer ratings, and boasted a collective score of 4.5 out of 5 stars.