The people of Guinea-Bissau are voting in parliamentary elections which are seen as a major test for the small West African nation's stability, BBC reports.
Four parties are expected to dominate the poll, including the new Republican Party for Independence and Development of President Joao Bernardo Vieira.
The former Portuguese colony has a history of coups, mutinies and uprisings since independence in 1974.
It has since become a hub for cocaine smuggling from South America to Europe.
The head of the European Union's observer team, Johan Van Hecke, told the BBC that the drug trade had been a major campaign issue.
"There have been some very inflammatory statements made by some political leaders accusing each other of being directly or indirectly involved in drug trafficking and for accepting money from people who are responsible for drug trafficking to fund their campaigns," he said.
The BBC's West Africa correspondent, Will Ross, says that in order to fight the drug traffickers, Guinea-Bissau desperately needs strong political leadership, which has been severely lacking as personal animosity has dominated politics.
There was hope that a power-sharing agreement signed last year would give President Vieira the political support to tackle the immense challenges facing the country, our correspondent says.
But when the deal broke down in July, there were reports of yet another attempted coup and the country has been in political limbo since, he adds.
Sunday's election is expected to be dominated by four parties - the Republican Party for Independence and Development (PRID), which was formed ahead of the election by former Prime Minister Aristides Gomes; the historically dominant African Party for the Independence of Guinea-Bissau and Cape Verde (PAIGC); the Social Renewal Party (PRS); and the Development, Democracy and Citizenship Party (PADEC).
One positive development of the campaign, according to Mr Van Hecke, has been the apparent absence of the army, upon which politicians have traditionally relied for power.
Our correspondent says that while international observers keep a close eye on the election, the drug barons are all too aware that the longer the political crisis continues, the easier it will be for them to increase their grip on Guinea-Bissau.
These elections matter, he adds, as the emergence of a narco-state would be a disaster in fragile West Africa.