The results of last week's election in Georgia cannot be explained, two US pollsters commissioned to carry out exit polls for opposition TV channels have said.
The reports by HarrisX and Edison Research came in the wake of widespread violations highlighted by election monitors in last Saturday's vote in the South Caucasus state.
The assessments by Edison Research and HarrisX will bolster the case made by opposition parties and the Georgian president, who have condemned the vote as rigged and stolen.
But the ruling Georgian Dream (GD) party insists the election was free and fair, although it acknowledges irregularities in "just a couple" of polling stations.
Edison Research said the 13-point difference between its own estimate and the 54% majority given to GD could not be explained by normal statistical margins and "suggests local-level manipulation of the vote".
A similar assessment from HarrisX revealed a discrepancy of more than 8% of votes "pointing to possible voting irregularities".
The election commission, which certified the official results, said it had come under "unfounded attack".
Opposition parties had billed the election as a choice between Europe and neighbouring Russia. President Salome Zourabichvili alleges the election was stolen as part of a "Russian special operation".
The EU, US and Nato have called for a transparent inquiry into widespread examples of irregularities.
Exit polls conducted by HarrisX and Edison Research on Saturday both gave the four opposition parties victory, well ahead of GD, which they said polled up to 42% of the vote.
In Marneuli, one of the towns where Georgian Dream (GD) conceded violations had taken place, two people have been arrested on suspicion of ballot-stuffing, including the town's deputy council leader.
HarrisX said that according to the election commission, GD won 80% of the vote in the town, whereas the pollster's own exit poll put the figure at 40%.
Edison said it had found deviation from expected results was most pronounced in rural areas, which were likely to have had the "most significant vote manipulation at the polling location level”.
The main opposition groups have called for further protests next Monday, after an initial demonstration attracted tens of thousands of people outside parliament on the main Rustaveli Avenue earlier this week.
Georgian observers have highlighted numerous violations in voting, including vote-buying, ballot-stuffing and voters' IDs being confiscated ahead of the election.
But the election commission has rejected allegations of bias and denied Zourabichvili's allegations of so-called carousel voting in Georgia's new electronic voting system, insisting it is impossible to vote more than once.
Public prosecutors summoned the president for questioning this week to prove her allegations of fraud, but she said that was their job.
