Protesters held up fake cameras in what they described as a bid to "spy" on the heavily secured military complex allegedly housing a snooping facility of the National Security Agency.
One protester wore a helmet bearing the phrase "democracy is watching you."
Police spokeswoman Andrea Loeb said some 450 protesters turned out for the rally today at the US military's so-called Dagger Complex on the outskirts of Griesheim town.
Allegations by leaker Edward Snowden about the NSA's wide-ranging spying activities targeting data and phone communication worldwide have caused concern in Germany, where protecting personal data is an especially sensitive topic.