Hundreds of gamers gathered at shops around the world ahead of its release at 00:00 GMT.
Retailer HMV has predicted more than 1m UK sales in the first week, 20% more than previous record holder Grand Theft Auto 4 (GTA4).
Online shop Amazon said the pre-order sales were 50% higher than GTA4.
Play.com say that in the run up to launch it was getting more than 150 pre-orders per minute.
The publishers of Modern Warfare 2, Activision, are cautiously predicting total sales of 3m across the UK whilst some analysts believe it could sell 10m copies globally in the run up to Christmas.
HMV stores in the UK opened at midnight to allow eager gamers to get their hands on the game as soon as possible.
crowds at Modern Warfare launch in London
Some gamers compared the London launch to a film premiere.
"We estimate that the game will exceed 1m UK sales in its first week, which would be a new record," said HMV spokesperson Gennaro Castaldo.
The firm also confirmed that its stock of the collectors edition, Modern Warfare 2: Prestige, sold out online pre-release.
"Because people haven't been able to play the game beforehand, it's really built their anticipation," said Neil Ashurst from retailer Game.
"I wonder how many people have taken the day off work today to play it."
Some people had managed to get their hands on the game early after shops broke the sales embargo.
Facebook group
Robert Kotick, CEO of Activision, described it as "one of the largest entertainment launches of any media of all time".
Analysts believe the game could sell as many as 5m units globally on its first day.
It is the sixth instalment in the Call of Duty series and gives players the chance to be a member of a military strike force that takes on a Russian ultra-nationalist terrorist group.
We're already seeing it bully other name-brand competitors into pushing their release dates back rather than risk going head-to-head in the same launch window
Scott Steinberg
Digital trends
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It sees the combat team travelling to Russia, Afghanistan, Kazakhstan, Brazil and into orbit, in an attempt to thwart the terrorists.
The game has drawn criticism because one section, prefaced by a warning screen, involves a player joining a massacre of civilians at an airport.
Labour MP Keith Vaz condemned the game saying he was "absolutely shocked" by its violence.
In response, publisher Activision said the scene was "not representative of overall experience".
Meanwhile, Labour MP Tom Watson has set up a Facebook group to defend games such as Call of Duty from their critics.
Price war
The first Modern Warfare debuted in 2007 and sold more than 12 million copies. The game won praise for its uncompromising story line and attention to detail.
Versions for the PlayStation 3, Xbox 360 and PC have gone on sale.
Screenshot from Call of Duty,Activision
MPs row over Modern Warfare
In the UK, a launch event was held at a cinema in London's Leicester Square, more usually the setting for film premieres.
Amazon said its pre-order sales for Modern Warfare 2 had been 50% higher than for Grand Theft Auto 4 - which until now had been the most successful release ever.
"We're already seeing it bully other name-brand competitors into pushing their release dates back rather than risk going head-to-head in the same launch window," said Scott Steinberg, of game and gadget website Digital Trends.
Since the Call of Duty franchise started in 2003, more than 28 million copies of the game's various incarnations have been sold.
In the UK, the console versions of the game are selling for about £54. The PC version is priced at about £35.
However, supermarkets and retailers have been advertising it a steep discount. Sainsbury's and Tesco are expected to offer it for about £26. Online stores have also cut the price of the title.
A limited edition of the game, called the Veteran Package, is also available for £99.
While many gamers eagerly awaited its release, fans of Modern Warfare have expressed disappointment that the PC version will lack some of the tools and tweaks available in earlier editions.
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