Regional body ECOWAS, in an unusually strong criticism of a member state, said on Tuesday it would not send a mission to observe the poll "because the preparations and political environment ... are adjudged by the commission not to be conducive for the conduct of free, fair and transparent polls."

It said a fact-finding mission had found "an unacceptable level of control of the electronic media by the party in power ... and an opposition and electorate cowed by repression and intimidation". 

Nearly 800,000 Gambians are registered to vote in the election pitting Jammeh against two opposition rivals.

Because of high rates of illiteracy, voters are casting their ballot by putting a glass marble into a coloured drum representing their candidate.

Analysts say Jammeh's election to a new five-year term is a foregone conclusion after 17 years of rule that began with a 1994 coup and has been marked by lethal crackdowns on protests, mass arrests of opponents and military reshuffles.