Guinea's presidential election was valid despite some logistical difficulties, European Union (EU) observers said on Tuesday in an apparent boost for a vote expected to return President Alpha Conde for another five years.

The EU statement came as clashes broke out over Sunday's election between supporters of the president and the opposition in at least two neighbourhoods of the capital Conakry, witnesses said. Security forces were seen heading to the area but it was not immediately clear how large the protests were.

The election results are yet to be officially announced. Early results announced by radio stations so far showed Conde with a sizeable lead, though a second round is possible.

"What I saw in terms of insufficient (voting material), lack of preparation and logistical and practical difficulties at the polling stations does not mar the validity of the election," said the head of the EU observer mission Frank Engels.

Nearly two-thirds of polls opened late because of insufficient materials and some did not receive voting booths at all, but representatives of the political parties were in most locations and counts were mostly transparent, Engels said.

Guinea, Africa's largest producer of bauxite, has a history of political violence, including at the 2010 election that brought Conde to power. Two people were killed and at least 33 were injured on Friday in clashes between Conde supporters and those of the main opposition leader Cellour Dalein Diallo.

Engels's comment was echoed by the head of the African Union monitoring group, which said the election was transparent despite some organisational difficulties.

Opposition candidates, including Diallo, called on Monday for the results of the election to be scrapped due to fraud.

Conde's election in 2010 ended two years of military rule. In 2009, security officials killed more than 150 pro-democracy protesters in a Conakry stadium and raped dozens of women.