Seven candidates are competing against President Alpha Conde, who is hoping to win a second term.
His main challenger, Cellou Dalein Diallo, has urged his supporters to vote despite the country's top court rejecting his plea for a delay.
The 2010 election saw a transition from military to civilian rule.
Since then Guinea has been badly hit by an Ebola outbreak and seen prices of key export bauxite - the raw material in aluminium - slump.
Opposition parties had wanted the vote postponed due to alleged anomalies in the country's electoral roll, but this has been rejected by the election commission.
At least three people have been killed in pre-election violence.
Mr Conde is favourite to win the election but a second round of voting is likely.
Guinea is to vote in a presidential election, with the build-up marred by violent clashes between opposition and pro-government supporters.