SAO TOME (Reuters) - Sao Tome and Principe voted in a presidential election on Sunday in which incumbent Manuel Pinto da Costa is looking to win a second term against four opponents including a former speaker of parliament.

The small island nation located in the Gulf of Guinea is a former colony of Portugal and has a semi-presidential system in which the prime minister is the head of government.

The country has 19 oil blocks in its exclusive economic zone and an additional joint exploration zone with Nigeria, but it is yet to find any commercially viable oil.

"Everything is proceeding normally," said Ambrose Quaresma, spokesman for the National Electoral Commission. Soldiers and police ensured security on Sunday and international observers were also present at polling stations, witnesses said.

Contenders include Evaristo Carvalho, vice president of the parliamentary ruling ADI party and a former speaker of parliament and Maria das Neves, a former first minister. Neves ran in 2011, coming fifth with 14 percent of a first round vote.

(Reporting by Ricardo Neto; Writing by Matthew Mpoke Bigg; Editing by Alan Crosby)