Seven phone-makers, including Sony, Samsung and Toshiba, and 16 operators including Orange, Vodafone and T-Mobile, are involved in the global launch.
Around 30 types of "Windows phones" with various designs will be available by the end of 2009 in more than 20 countries.
"We have done a lot of work on the user interface, we simplified the user interface," Ballmer told a news conference at Microsoft's new French headquarters.
"We have taken the Internet Explorer browser technologies, and we rebuilt them for the first time for these Windows phones. So you can get the same experience on these phones that you will get on your Windows PC," he said.
The new mobile operating system was launched simultaneously in France and New York.