The Pacific nation has marked Wednesday as a public holiday for its election, with the race shaping up as a contest between two former coup leaders.
Bainimarama has been in charge since he instigated a coup in 2006 and took over the prime ministership the year after.
He took FijiFirst to a democratic election in 2014 and won but his majority was cut to 50.02 per cent at the 2018 poll.
Bainimarama said he would "of course" respect the outcome of the democratic election when he cast his vote but concerns over a possible coup remain in a nation marred by takeovers.
He maintains he's confident he and his FijiFirst government will be able to retain power.
Sitiveni Rabuka expressed doubt the prime minister would accept defeat.
Rabuka heads the People's Alliance party and served as a former prime minister, initially taking power through a coup in 1987.
He said there was little prospect of a coup, even if the prime minister decided to contest the result, after Fiji's military commander told his soldiers to respect the outcome of the election and said anything less would be an affront to democracy.
The Pacific nation has been marred by four coups since it gained independence in 1970.
"I accepted my defeat in 1999 and I hope he can do that," Rabuka said.
"We cannot live forever. We cannot rule forever.
"A successor from an opposition party should be accepted. It is a normal process in democratic systems."
However, some voters remain concerned about potential repercussions if they speak out against the government.
One, who gave his name as Jack, told AAP he wanted a change in government but both frontrunners were coup leaders.
"They're both thieves. They took the government without permission," the 50-year-old said.
"I want a change in government but when we are given a choice between two thieves, what hope is there for the future?"
People's Alliance party is working with the smaller National Federation Party to expel the incumbent FijiFirst government.
Rabuka and NFP leader Biman Prasad believe there is a mood for change.
"I think it's going to go very, very well for us," Rabuka said on Wednesday.
Lines stretched more than 300 metres at some polling stations an hour after voting opened.
Voting is not compulsory but Fijians have been given a public holiday and encouraged to cast their ballot.
Supervisor of Elections Mohammed Saneem said turnout was not looking promising, with only 27 per cent of eligible Fijians casting their ballot before his midday media update.
"Let's spend the next six hours getting our family members to get out and vote," he said.
Bainimarama and FijiFirst are running on their record during COVID-19 and a strong economy. He's also spruiked free education, greater access to medical care and better infrastructure under his government.
Rabuka and opposition parties are arguing Fiji's infrastructure and essential services are dilapidated.
They say poverty and inequality have increased while freedoms have been quashed by a prime minister they brand a dictator.
This article was made possible through the Melbourne Press Club's Michael Gordon Journalism Fellowship Program.
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