Mobs shouting charges of fraud stoned election offices and smashed store windows Monday during Guayana's first balloting in eight years.
Supporters of President Desmond Hoyte, who had trailed in surveys, complained of being refused ballots at polling places.
Balloting in the South American nation was being monitored by more than 100 foreign observers, including former President Jimmy Carter.
Guyana, the size of Kansas, has about 750,000 inhabitants, most of whom are poor.
The rioting occurred after thousands of Guyanans, most of them black backers of Hoyte's socialist People's National Congress, massed outside national election headquarters to protest.
"We want to vote!" they chanted, pressing against the gates of a 5-foot steel fence ringing the building in downtown Georgetown, capital of this former British colony.
Youths pelted the three-story building with rocks. Hundreds swarmed down commercial Regent Street, smashing windows.
There were no immediate reports of injuries.
Political support is divided largely along racial lines, with descendants of East Indian indentured servants backing Cheddi Jagan's People's Progressive Party and descendants of black African slaves supporting Hoyte's party.