By Felix Light and Lucy Papachristou

Three Georgian opposition parties who allege last weekend's election was rigged called for streets protests next Monday as an exit pollster said the official result, which gave victory to the ruling Georgian Dream party, was "statistically impossible".

Global market research and data firm HarrisX, which carried out an exit poll for Georgia's weekend parliamentary election, said on Thursday that its analysis showed discrepancies in the results reported by the Central Election Commission (CEC) that could not be statistically explained and pointed to possible voting irregularities.

HarrisX's poll - commissioned by a Georgian opposition television station, Mtavari Arkhi - showed the combined four main opposition parties on course for a parliamentary majority in Saturday's election.

"We will gather on Monday at 7 p.m. ... and tell you there our plan of action. In detail, how at home or abroad, within the legal framework, peacefully, but via organization and with a concrete result, we will continue our protest," Ana Dolidze of the Strong Georgia opposition bloc told a press conference of the three opposition parties.

Georgian Dream, whose billionaire founder Bidzina Ivanishvili is seen as the country's most powerful man, won nearly 54% of the vote in Saturday's election, the election commission said.

However, monitoring missions, including the Organisation for Security and Cooperation in Europe, pointed to severe allegations of voting violations which Western countries said must be fully investigated.

Georgian opposition leaders said sustained protests were key to their plan to challenge the election result.

"This is the most direct and ancient form of democracy, where a given government refuses to listen to its own people at the ballot box," Tina Bokuchava, leader of the United National Movement opposition party, told Reuters.

The analysis of the U.S.-based firm lends powerful support to complaints by President Salome Zourabichvili and opposition parties that the election was stolen. On Monday, thousands rallied outside the parliament building in Tbilisi against the results.

Many in Georgia and the West see the election as an important turning point that could determine whether the Caucasus nation shifts back into Russia's orbit under Georgian Dream or moves closer to its stated goal of joining the European Union.

The CEC has insisted the vote was free and fair. Prosecutors have launched an investigation and called on Zourabichvili, an opponent of Georgian Dream, to provide proof of her allegations of fraud - something she has said is their job, not hers.

"The analysis of our Exit Poll calibrated by final CEC information raises questions over the final vote count released by the Central Election Commission of the Republic of Georgia," HarrisX said.

"We see statistically unexplainable discrepancies amounting to over 8 percent of the total vote, or at least 172,523 raw votes, across a minimum of 27 districts."

It said the discrepancy "cannot be explained by statistical variance, pointing to possible voting irregularities."