SAN FRANCISCO — TODAY, we’re filing a lawsuit against the National Security Agency to protect the rights of the 500 million people who use Wikipedia every month. We’re doing so because a fundamental pillar of democracy is at stake: the free exchange of knowledge and ideas.

Our lawsuit says that the N.S.A.’s mass surveillance of Internet traffic on American soil — often called “upstream” surveillance — violates the Fourth Amendment, which protects the right to privacy, as well as the First Amendment, which protects the freedoms of expression and association. We also argue that this agency activity exceeds the authority granted by the Foreign Intelligence Surveillance Act that Congress amended in 2008.

Most people search and read Wikipedia anonymously, since you don’t need an account to view its tens of millions of articles in hundreds of languages. Every month, at least 75,000 volunteers in the United States and around the world contribute their time and passion to writing those articles and keeping the site going — and growing.

On our servers, run by the nonprofit Wikimedia Foundation, those volunteers discuss their work on everything from Tiananmen Square to gay rights in Uganda. Many of them prefer to work anonymously, especially those who work on controversial issues or who live in countries with repressive governments.