(Add voting started, Gallup poll)

By Jorge Pineda

SANTO DOMINGO, May 15 (Reuters) - Dominican Republic President Danilo Medina was poised to win the first round of a presidential election on Sunday, with polls showing voters crediting him with economic growth and social projects, while less concerned about accusations of graft.

Voters lined up outside polling stations in schools before they opened at 6 a.m. AST (1000 GMT) and by mid-morning 15 percent of eligible voters in the Caribbean's largest economy had cast their ballots.

A Gallup-Hoy poll published on April 25 showed Medina winning 63 percent of the vote, enough to avoid a second-round runoff in June. His ratings have been climbing in recent months, at the expense of his nearest rival, businessman Luis Abinader.

Voting intentions barely reach 10 percent for the remaining six candidates combined, including the first two women running for the presidency in a Dominican election.

"We are now better off than four years ago," said Lisbelis Acosta, 24, after voting in Santo Domingo. She said the government had improved health and education services.

A left-of-center economist, Medina has had high popularity ratings during the latter part of his four-year term in the country of 10.4 million. Electoral rules were changed to allow him to run for a second consecutive term.

"I won't be satisfied until progress reaches everyone, when growth means a table full of food for everyone," Medina, 64, said at a rally to close his campaign on Thursday.

Medina's Dominican Liberation Party has been continuously in power since 2004.

Abinader promises to double down on social spending and reduce crime. The challenger has also focused on allegations of corruption related to a power plant awarded to Brazilian engineering conglomerate Grupo Odebrecht.

Medina's campaign chief, João Santana, returned to Brazil in February to face charges Odebrecht had paid him funds siphoned from Brazil's state oil company Petrobras in offshore accounts to finance the 2014 election campaign of suspended Brazilian President Dilma Rousseff.

Medina has yet to refer to the Petrobras scandal, but he did admit the Brazilian political strategist was his top adviser. Santana has called the allegations against him "baseless."

With the fastest growing economy in Latin America in 2014 and 2015, the Dominican Republic is wealthier than Haiti, its poor neighbor on the island of Hispaniola.

Medina has overseen the repatriation of tens of thousands of people with roots in Haiti. The policy is popular at home but condemned by human rights groups.