BEIJING—Chinese authorities detained dozens of political activists after an anonymous online call for people to start a "Jasmine Revolution" in China by protesting in 13 cities—just a day after President Hu Jintao called for tighter Internet controls to help prevent social unrest.

Only a handful of people appeared to have responded to the call to protest in Beijing, Shanghai and 11 other cities at 2 p.m. Sunday, a call first posted on the U.S.-based Chinese-language news website Boxun.com and circulated mainly on Twitter,...