Microsoft chief executive Steve Ballmer on Tuesday said the Windows 8 operating system was off to a "stunning" start with four million upgrades downloaded since its release on October 26.

"In just the last three days, we have sold four million Windows 8 upgrades," Ballmer said while kicking off a BUILD Conference for developers at the software giant's headquarters in Redmond, Washington.

"The level of embrace from enthusiasts is very, very high."

Ballmer added that tens of millions of businesses have also switched to the latest version of Windows.

Analysts have depicted Windows 8 as a bold bet for Microsoft since it dramatically changed the long-familiar user interface to adapt to touch-screen mobile devices increasingly popular in modern lifestyles.

"The level of enthusiasm seen for the new products has really been stunning," Ballmer told the thousands of developers at the sold-out event.

Globally popular one-to-many text messaging service Twitter and online file sharing hit Dropbox are among applications being tailored to Windows 8, Ballmer told third-party software developers at the gathering.

Microsoft kicked off sales of its revamped Windows 8 system and Surface tablet Friday as it ramped up efforts to compete in a market shifting rapidly from PCs to mobile devices.

Microsoft on Monday set out to win over iPhone and Android gadget devotees with Windows 8 smartphones, new devices that emphasize individualism and unify digital lives in the Internet cloud.

The global rollout of Windows 8 phones, set to begin in Europe this weekend, is the final piece in a Microsoft operating system transformation aimed at harmonizing the technology titan's software and hardware for mobile lifestyles.

The operating system has a user interfaced based on "tiles" that can be personalized with people's pictures, applications, games, music and more.

Microsoft corporate vice president Joe Belfiore said that Microsoft was out to "re-invent" a smartphone experience that has for years basically consisted of a locked screen that opens to rows of icons.

"We decided not use that tired old metaphor and came up with our own way that puts people at the center of the experience," Belfiore said with a slight jab at iPhone.

The BUILD conference was also intended to inspire developers to make fun, hip, or functional "apps" for devices powered by Windows 8 software.

Microsoft's online shop is stocked with more than 120,000 applications tailored for Windows 8 phones and the number is growing, according to Belfiore.

Analysts have long contended that desirable "apps" are crucial to the success of smartphones.

Analysts said Microsoft is trying to walk a fine line by keeping hundreds of millions of Windows users worldwide while accelerating efforts to compete in the mobile world dominated by Apple and Google.

Microsoft planned a massive advertising campaign for Windows 8.