"We have found many people queuing in front of polling stations and everything is going well," Emilie Beatrice Epaye, an election observer from the Central African Economic and Monetary Community (CEMAC) told reporters.
The vote was initially scheduled for November, but was brought forward following a decree by President Teodoro Obiang, who gave no reason for the change.
Around 320,000 people were eligible to vote in the elections on Sunday. But opposition parties said they would not participate in the polls amid reports that security forces had surrounded the headquarters of Equatorial Guinea's largest opposition party, Ciudadanos por la Innovacion.
Earlier on Sunday, the Spanish government said it was "worried" about news of such incidents in its former colony.
36 years in power
President Teodoro Obiang of the Democratic Party of Equatorial Guinea was running as the head of a 10-party coalition. He was expected to defeat his six challengers in Sunday polls.
Obiang came into power in 1979 after toppling his uncle, Francisco Macias, in a coup. Macias had ruled the country since 1968, when it won independence from Spain. The leader executed thousands of people and forced a quarter of the population into exile.
Since then, Teodoro Obiang has been ruling Equatorial Guinea, winning the last elections in 2009 with 95 percent of the vote. His regime has been accused of suppressing the media and the opposition, rampant corruption and seizing profits from the country's rich oil resources.
Equatorial Guinea has been one of the fastest growing economies in Africa over the last decade. However, falling oil prices have slowed growth and about 77 percent of the country's 1.2 million people live in poverty, according to World Bank figures.
mg/jlw (AFP, dpa)