Gamers across Europe got their hands on Halo 3 at midnight on Tuesday, one of the most anticipated and heavily marketed titles in history.
In the UK more than 1,000 shops opened at midnight so gamers could get their hands on the title.
Many gamers started queuing outside shops in the afternoon to ensure they got hold of a copy.
The Xbox 360 game is Microsoft's key weapon in the console wars with Sony and Nintendo.
Simon Soffe, a spokesman for retail chain Game which opened 150 shops, said: "Half of them seemed to be going home to play on their own and the other half were planning to go online and play against other people."
Microsoft hopes day one sales will top £70m ($140m), more than the opening takings of any movie in history.
However, sceptics point out that video games cost upwards of £40 ($80), while cinema tickets are much less.
In New York, about 500 people turned out for the midnight launch.
Gamer Alex Escobar, who was one of the first in the queue, told Reuters news agency: "It is worth it. It is time to finish this fight," echoing the tagline for the science fiction game.
Shane Kim, corporate vice president of Microsoft Game Studios, said the hoped for success of Halo 3 was an essential element in the competitive console market.
He said: "Halo 3 is the biggest franchise for Xbox. The game is going to drive a lot of Xbox 360 sales and Xbox Live subscriptions this Christmas."
Microsoft needs Halo 3 to boost sales of the Xbox 360; despite investing billions of dollars into the Xbox project it has yet to see any meaningful profitable return
"It's far too early to say what the financial return will be for our investment," Mr Kim told the BBC News website.
Microsoft hopes to make a profit on its Xbox in this financial year.
"If we can't make a profit in the year Halo 3 comes out, then when will we?" said Mr Kim.
More than a million people pre-ordered the game, which is the concluding part of a science fiction trilogy that tells the story of a super soldier, called Master Chief, who is leading the fight to save humanity from an alien collective, called the Covenant.
The game has become a major entertainment franchise in recent years - with spin-off games, clothing, novels and action figures all available.
The Lord of the Rings director Peter Jackson is working with Halo's makers, Bungie studios, on a series of Halo-related interactive episodes.
He has also agreed to produce a movie based on the Halo series, which is currently on hold following financial wrangles over the cost of the film.
"It's not necessarily going to move a lot of new systems like the first Halo did," said Dan Hsu, editor-in-chief of US games magazine EGM.
"At the same time, with all the marketing blitz and hype, consumers will be out there," he said, "and if they are thinking video games, they are thinking one of two things: Halo or the Wii."
Web reviews of the game have begun to emerge with most posting very high scores.
Game site Eurogamer wrote: "Hype machine aside... what we find in Halo 3 is quite simply this - the best game yet in one of the best FPS (first person shooter) franchises of the era."
All three of the leading games consoles rely on exclusive franchises to drive sales. PlayStation 3 has titles such as Metal Gear Solid 4 and Killzone 2, while Nintendo has Metroid Prime 3 and Zelda.
Microsoft has spent a reported $10m (£5m) on promoting the game, with a series of costly TV adverts, as well as signing deals with food and drink firms in the US.
For Bungie, the game's release is the culmination of three years work.
Brian Jarrard, Bungie's director of franchise and community affairs, said the company had not felt under pressure from Microsoft, which owns the studio.
"We feel the most pressure ourselves. It's always been about driving ourselves to do great work. For the most part we don't feel the pressure from Microsoft executives breathing down our necks.
"The fans' expectations are incredibly high. We gave them a bitter sweet ending at the end of Halo 2 and we're pretty confident they will be happy with Halo 3 and that it was worth the wait."