Lima police chief Gen. Luis Praeli said 15 people were arrested during Saturday's unrest.
It was the fourth popular demonstration this month in Lima—none have been especially large—and comes a day before Peru's independence day, when Humala is to give a state-of-the-union address.
Discontent with Humala's two-year-old rule includes repudiation of Congress' voting to name 10 officials to bodies including Peru's high court that lawmakers later revoked.
Doctors and nurses at public hospitals have also been on strike.
Humala's approval rating is now at 33 percent, his lowest since taking office.