People who showed up for the Saturday event, one of at least 50 scheduled around the world using social networking Web sites, said officers were stationed at the park and confiscated pillows prior to the brawl's planned 4 p.m. start time, the Detroit News reported Monday.
One attempted pillow fighter said he was told by an officer that about 5,000 pillows had been collected as of 4 p.m. Saturday.
Detroit Police spokesman James Tate said the event posed cleanup issues for the city, as large-scale pillow fights tend to leave large amounts of feathers behind, and officers were concerned that some people who didn't want to participate could be hit by mistake.
"They took my pillows but let me keep my cases," Michael Davis, 32, told the News. "They told me I needed a permit. I can understand."
However, some of those whose pillows were taken were less understanding, the newspaper said.
"I am furious," said Elida Quesada, 23. "It (a pillow fight) is so silly and childlike. It would have been fun. It seems like everything that is fun is illegal."