Gambians are voting in presidential elections condemned by the West African regional body, Ecowas, as lacking legitimacy.
Because of high rates of illiteracy, voters are casting their ballot by putting a glass marble into a coloured drum representing their candidate.
President Yahya Jammeh, who seized power in 1994, is seeking re-election.
Ecowas said the poll would not be free and fair because of high levels of intimidation by the ruling party.
"The preparations and political environment for the said election are adjudged by the commission not to be conducive for the conduct of free, fair and transparent polls," the 15-state West African bloc said in a statement ahead of the poll.
It said its investigations found "an opposition and electorate cowed by repression and intimidation".
It would, therefore, not send election observers to The Gambia, Ecowas said.
Observers from the African Union (AU), Commonwealth and the Organisation of Islamic Conference (OIC) are, however, monitoring the poll.
AU observers said they were so far satisfied with the electoral process, reports the BBC's Umaru Fofana from the capital, Banjul.
The chairman of Gambia's Independent Electoral Commission, Mustapha Carayol, reacted angrily to Ecowas's decision.
"The allegations are all lies," he told the BBC.
Mr Jammeh, 46, faces Ousainou Darboe, leader of the United Democratic Party (UDP), and Hamat Bah, who is being backed by a coalition of four opposition parties.
Hundreds of people in Banjul queued before polls opened at 0700 GMT to vote, the AFP news agency reports.
A 27-year-old voter, Amie Khan, was one of the first to cast her ballot at the city's Kombo North polling station - sending a clang ringing out as her marble hit the bicycle bell inside a dark green drum bearing a picture of her candidate, AFP says.
"I voted for Jammeh because thanks to him I was able to attend university here," she said.
The Gambia has used drums and marbles, instead of ballot papers and boxes, since 1965 because of high levels of illiteracy.
Votes are cast in a booth to ensure secrecy.
At a polling station in Serrekunda, near Banjul, unemployed voter Musa Sanneh said he had cast his ballot for the opposition.
"Seventeen years of misrule is enough so Jammeh must go," he is quoted as saying.
Mr Jammeh has won three widely criticised elections since seizing power in a bloodless coup 17 years ago.
Ahead of the presidential poll in 2006, Ecowas urged the opposition not to boycott the elections and endanger the "democratisation of the country".
Mr Jammeh's government has been criticised by international rights groups for its attitude to civil liberties, especially freedom of the press and the detention and disappearance of journalists.