* Observers say problems with voting could question some results
* Opposition calling for the annulment of long-delayed vote
* Six of eight problem districts won by the opposition (Adds details)
By Saliou Samb
CONAKRY, Oct 9 (Reuters) - Irregularities took place in a number of districts in Guinea's parliamentary election last month, possibly harming the credibility of the results, international observers said.
Opposition groups have demanded the annulment of the Sept. 28 vote and have pulled out of U.N.-brokered talks with President Alpha Conde's government after accusing it of rigging the election.
About 50 people were killed in political violence before the poll, which was aimed as completing Guinea's move back to democracy following a 2008 coup.
Observers from the United States, France, the European Union, the United Nations and West African regional bloc ECOWAS said they had identified problems with voting in eight electoral districts. Six of these districts were won by the opposition.
"Shortcomings and irregularities were observed in a certain number of constituencies preventing the inclusion of a significant number of ballots and therefore possibly bringing into question the fairness of some results," they said in statement issued on Tuesday.
The group urged all parties to make use of legal channels for any challenges.
Moustapha Naite, spokesman for Conde's ruling RPG party, said the opposition had committed "massive fraud" in some districts. The RPG would take its case to court, he said.
Sidya Toure, leader of the opposition UFR party, denounced what he said was the arbitrary closure of voting bureaux in opposition areas and the introduction of fraudulent voting tallies by government representatives at some voting stations.
"Everyone knows that the only means of response recently has been on the streets," political analyst Madani said. "I fear that things are going to get out of control."
The EU observer mission has already pointed to flaws, including the use of outdated voter rolls, in the organisation of the poll, which had been delayed by political haggling.
No single party was expected to win an overall majority in the 114-seat assembly. Partial results show Conde's ruling RPG party with a slight lead over opposition leader Cellou Dalein Diallo's UFDG and former prime minister Toure's UFR.
But opposition groups rejected those results last Thursday and pulled their representatives out of the electoral commission the following day. They have demanded the cancellation of the elections due to what they say was widespread fraud.
CENI has still not published results from at least three of the 38 voting districts.
Guinea slashed its economic growth forecast for 2013 to 2.9 percent from 4.5 percent after the violent protests and political paralysis hit investment in the mining sector.
(Reporting by Saliou Samb; Writing by Joe Bavier, Editing by Gareth Jones and Angus MacSwan)