The Gambia has banned the internet and international phone calls as presidential elections are held in the West African state.
Officials have also banned demonstrations to prevent unrest after the elections.
Estate agent Adama Barrow is challenging President Yahya Jammeh, who says divine intervention will give him a fifth term.
The Gambia has not had a smooth transfer of power since independence.
The BBC’s Umaru Fofana in the capital, Banjul, says long queues have formed at polling stations.
The Gambia has a unique way of voting for the president.
Instead of using ballot papers, voters put a marble into a coloured drum for their candidate.
The system is aimed at tackling illiteracy and preventing rigging.
In the 2011 election, only two votes were declared invalid after people placed their marbles on top of drums.
Observers say Mr Barrow, who, according to British news reports, was once a security guard in the UK high street store Argos, has generated fresh enthusiasm among opposition supporters.
Human rights groups accuse Mr Jammeh, who has in the past claimed he can cure Aids and infertility, of repression and abuses.
Several previously better-known opposition leaders are in jail after taking part in a rare protest in April.
Observers from the European Union (EU) and the West African regional bloc Ecowas are not attending the vote.