* Move follows results handing ruling party most seats
* Vote meant to complete transition back to civilian rule
* Observers identified problems in some districts
CONAKRY, Oct 19 (Reuters) - Guinea's main opposition parties accused the ruling party on Saturday of massive fraud in last month's parliamentary election and said they would call on the Supreme Court to annul the vote.
The statement was the first official reaction to the complete results issued on Friday, which showed the President Alpha Conde's ruling party won 53 seats but fell just short of an absolute majority in parliament.
The Sept. 28 vote, which is meant to complete the return to civilian rule but was more than two years late, has been dogged by wrangling between parties, triggered deadly street protests and dented investor confidence in the iron and bauxite-rich state.
"We have decided to file a complaint (at the Supreme Court), with all the necessary proof, calling for the complete annulment of the vote," said Aboubacar Sylla, a spokesman for the umbrella of opposition groups.
The coalition includes Cellou Dalein Diallo's UFDG, which came second with 37 seats, Sidya Toure's UFR, which won 10 seats and two other minor parties that secured a pair of seats each.
"What we are seeing a complete farce brought about by massive fraud organised by the ruling party," Sylla added.
Opposition parties had earlier called for the vote to be cancelled during the counting process. They also threatened more protests. About 50 people were killed in demonstrations over election preparations earlier this year.
Conde's camp has already said it will challenge in the Supreme Court the results from some districts where it says the opposition rigged the outcome.
Earlier this month, 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 a number of districts, potentially harming the credibility of these results.
But diplomats, led by the United Nations, have sought to restore confidence in the process and, in particular, the election commission, which is deeply divided.
Conde's RPG party needs the backing of another four seats in parliament to secure a majority so a period of deal-making is expected after results are confirmed.
While an important last step in the tortuous return to civilian rule following a 2009, the vote is also seen by many politicians in Guinea as warm-up for the presidential election in 2015, when Conde's first 5-year term comes to an end. (Reporting by Saliou Samb; Writing by David Lewis)