Hundreds of Venezuelan police and National Guard troops broke up an opposition march Friday with volleys of tear gas and water cannon blasts.
Thousands of opponents of President Hugo Chavez retreated as clouds of gas spread out over an avenue in downtown Caracas.
City health official Ivon Lamprea said at least 15 people suffered minor injuries, including a police officer whose hand was injured and a protester who was hit in the face by an object. She said the rest were treated for inhaling tear gas.
People in surrounding buildings hurled glass bottles at officers, and some beat on pots and pans to protest the police response.
Officers pursued demonstrators into side streets and fired tear gas at one group led by Caracas Mayor Antonio Ledezma, a prominent Chavez opponent.
The violence broke out as thousands of opponents and supporters of Chavez held separate May Day marches in the Venezuelan capital.