Activision's Call of Duty 4 and Microsoft's Halo 3 have been trading the first and second slots on Microsoft's Xbox Live activity chart for the past few months.

But Aaron Greenberg, director of product management for Xbox 360 and Xbox Live, said that GTA IV, which just released on Tuesday, is the new king of the hill on Xbox Live.

"We had roughly 1 million users in the first 24 hours [of GTA IV's release] go online with the game on Xbox Live," said Greenberg.

The online attraction of GTA IV is staggering, considering that until now, GTA has always been focused on the single player, offline experience.

"It's been the number one game on Xbox Live since its launch, so it's knocked off Halo 3 and Call of Duty 4," said Greenberg. "At launch, our data shows that out of all of our members on Live, the majority of them were playing GTA IV.

"With a community of over 10 million members, that's pretty rare."

Microsoft measures the number of unique users per game on Xbox Live. Aside from multiplayer online sessions, single-player sessions are also taken into account in the Live activity rankings if the console is connected to the service.

Greenberg also said that 2 million GTA IV achievements were unlocked in the first 24 hours of the game's release. In addition, over 15 million Gamerscore points were added to users' accounts in the first day.

1.5 million cross-game invites also occurred during that period.

Microsoft has been keen to leverage GTA IV's strong online multiplayer component by including a one-month Xbox Live gold subscription with all GTA IV launch copies. The firm hopes the game will drive Xbox Live adoption. Gold subscriptions cost $50 for a full year.

"Our teams have been doing a lot of work in advance of this game, in fact, months. The operations team has been working on scaling our infrastructure around building and increasing our server capacities," he added.



See more about Xbox 360 and initial GTA IV sales for the console here.