The Iranian regime has developed, with the assistance of European telecommunications companies, one of the world's most sophisticated mechanisms for controlling and censoring the Internet, allowing it to examine the content of individual online communications on a massive scale.

Interviews with technology experts in Iran and outside the country say Iranian efforts at monitoring Internet information go well beyond blocking access to Web sites or severing Internet connections.

Instead, in confronting the political turmoil that has consumed the country this past week, the Iranian government appears to be engaging in a practice often called deep packet inspection, which enables authorities to not only block communication but to monitor it to gather information about individuals, as well as alter it for disinformation purposes, according to these experts.

The monitoring capability was provided, at least in part, by a joint venture of Siemens AG , the German conglomerate, and Nokia Corp. , the Finnish cellphone company, in the second half of 2008, Ben Roome, a spokesman for the joint venture, confirmed.

The "monitoring center," installed within the government's telecom monopoly, was part of a larger contract with Iran that included mobile-phone networking technology, Mr. Roome said.