Centre-right incumbent Leonel Fernandez was the favourite among seven candidates bidding Friday for the presidency in this Caribbean country, where calm prevailed under the watchful eyes of 270 election observers, dpa reported.
More than 5.7 million voters were entitled to vote at 13,000 polling stations which opened at 6 am (1200 GMT) and were set to close by 2400 GMT.
Jose Octavio Bordon, head of the mission of observers from the Organization of American States (OAS), called upon Dominicans to await calmly the preliminary results that electoral authorities were expected to release later Friday.
"The Organization calls upon political parties and citizens in general to await patiently and with confidence for the results," Bordon said in a press conference.
He stressed that the election was marked by "calm and enthusiasm" despite violent outbursts in some polling stations, which left at least five people injured.
"The mission calls (Dominicans) to continue to vote in the same atmosphere of respect shown until now," Bordon added.
He said that 70 OAS observers were to visit 500 polling stations in the country's 32 provinces. Bordon noted that all of the 13,000 polling stations in the Dominican Republic are believed to fulfil the requirements, although some suffered delays in the start of voting due to logistic faults.
He added, however, that OAS has received complaints about attempts to buy identity documents on election day, although he said there were only a limited number of reports.
Beyond the OAS mission, some 200 international observers from other organizations were on hand, were in alongside 42,000 police officers supported by eight helicopters.
Earlier, electoral authorities called upon Dominicans to exercise their right to vote and noted that the country remained calm despite an incident linked to the election hours earlier.
On Thursday, supporters of Fernandez's Party for Dominican Liberation (PLD) and of the social democratic Dominican Revolutionary Party (PRD) of his main rival Miguel Vargas clashed, and four PLD supporters were injured. Still, the current campaign has been described as one of the least violent in the Caribbean country's history.
Vargas voted early in Santo Domingo, while incumbent hopeful President Fernandez was planning to cast his ballot in the afternoon.
"Beautified opinion polls say Fernandez will win, but the true polls say we will win. We are now at the most important opinion poll, on May 16 (election day) and we will win this one," Vargas told reporters after casting his ballot.
Most opinion polls have said that Fernandez is likely to get at least 55 per cent of the votes, granting him re-election and a further four-year mandate without need for a runoff. Vargas is expected to get around 37 per cent of the vote, according to opinion polls.
Fernandez, 54, already held the Dominican Presidency 1996-2000 and 2004-2008.
The Dominican Republic - a country where 42 per cent of its 9.1 million people live in poverty - has a bicameral Congress in which both houses are currently controlled by Fernandez's party. The country is set to hold legislative elections in 2010.