Voters in the town of Barahona, about 110 miles southwest of the capital Santo Domingo, said a small group of rival supporters of the two main candidates were responsible for the deaths on Sunday.
The country is holding a presidential election that is expected to oust President Hipolito Mejia.
Elsewhere in the Caribbean nation of 8.5 million people, election observers said voting was largely going smoothly with a large turn-out expected.
Opinion polls have indicated that candidate Leonel Fernandez of the opposition centrist Dominican Liberation Party will win the presidency for a second time in his career.
Leonel Fernandez promises
He needs at least 50% to avoid a run-off vote on 30 June.
Among the several other candidates only one, Christian Democrat Eduardo Estrella, has a significant poll indicating about 10% support.
Fernandez was president from 1996-2000 and has promised to restore the country to the prosperity it enjoyed under his watch, when annual economic growth averaged more than 7%.
Leading the Dominican Revolutionary Party, Mejia has blamed the current economic crisis on factors beyond his control.
The economy began its slide after the 11 September 2001 attacks on the United States and was further damaged by the collapse of a leading private bank a year ago.