Organizers of the "Freedom Not Fear" event pegged the turnout in Berlin's Alexanderplatz at 20,000 throughout Saturday's protest, during which speakers denounced the U.S. National Security Agency and Britain's GCHQ program of sifting through databases of people's email, online chat and Internet browsing histories without prior court authorization.
Other crowd estimates varied, from the official police estimate of 4,800, to 15,000 reported by Der Spiegel. Protesters carried signs reading "Anonymity is not a crime," "Monitoring destroyed democracy" and "Freedom of the press needs information protection."
Organized by 80 sponsors -- including civil society groups, trade unions, the Chaos Computer Club and the FDP, Left, Green and the Pirate parties -- the event was hailed by its backers as a "huge success" as speakers called to abolish German cooperation in the NSA monitoring of cellphone call data and online searches.
Speakers called for the defeat of German Chancellor Angela Merkel in the Sept. 22 federal elections. The NSA spying scandal has become a key issue in the run-up to the election, with Merkel's conservatives still leading at the polls.
Ronald Pofalla, Merkel's chief of staff, insisted last month the U.S.-British data searches are not contrary to German laws designed to protect privacy and fears of mass data scans were unfounded, saying, "In Germany there are no infringements of fundamental rights by the millions, as has been continuously, falsely alleged."
U.S. Internet activist Jacob Appelbaum spoke at the start of the demonstration and was interrupted repeatedly by cheers as he called on Germans to defend themselves against surveillance, the weekly Die Zeit reported.
"You need to kick out the Merkel government," he said, while also apologizing to the German crowd for the U.S. administration of President Barack Obama for allowing the NSA program to continue.
Gerd Billen of the Federation of Consumer Organizations stressed the importance of fighting against surveillance and translating the outrage into action.
"It's about our right to self-determination," he told the crowd. "This fundamental right is trampled underfoot every day."
Blogger Anne Roth called on attendees to fight the feeling of powerlessness, telling them they can do something against surveillance.
"We need secure software that is easy to use," she said. "It's the job of the federal government to ensure that our fundamental rights are protected, but that isn't happening."
Kai-Uwe Steffens of the Working Group Against Data Retention passionately criticized intelligence agencies such as the NSA over "unrestrained spying on phone calls and Internet traffic worldwide," and denounced the German government for remaining idle, Deutsche-Welle reported.
"This affair will be finished on the day when we are no longer monitored, and not a day earlier," he said.
The German Green Party has called for European Court of Justice proceedings against Britain over the spying allegations and has urged European Union Council of Ministers to suspend ongoing negotiations on a free trade agreement with the United States until there is further clarification on the scope of the data sifting.