Voters post their marble into the different coloured drums

Voters enter a booth and pop a clear glass marble into one of three drums representing the candidates, instead of a putting a ballot paper into a box.

As the marble falls into the drum, it hits a bell so officials can tell if anyone votes more than once.

"It's a unique system introduced in 1965 because of Gambia's high illiteracy," Gambia's chief electoral officer Kawsu Ceesay told the BBC.

The bell resembles a bicycle bell so bicycles are banned from around polling stations to avoid any confusion.

"Three drums representing the three presidential candidates will be in the compartment attached to one another so they can't be lifted to see which is heavier," he says.

The system also makes the counting process much more transparent

Chief electoral officer Kawsu Ceesay

"The drums are painted in the colour of the candidate's party and have their photograph and party symbol."

Incumbent President Yahya Jammeh's drum is green and his rivals Ousainou Darboe and Halifa Sallah are yellow and grey respectively.

Sand or sawdust is also put into the bottom of the drum before it is inspected by party agents and shut with numbered seals so that a second sound is not heard.

Afterwards voters have their finger dipped in indelible ink.

Marbles have to be posted through a pipe at the top of the drum and those left on top are regarded as spoilt ballots.

"The system also makes the counting process much more transparent," Mr Ceesay says.

The marbles are placed into trays with either 200 or 500 holes - similar to a solitaire board - which makes it easy for officials to verify numbers.