KIEV (Reuters) - Ukraine's State Security Service SBU accused Russia on Thursday of meddling in the electoral process in Ukraine by creating illegal structures to help guarantee victory for a certain candidate.

Ukraine holds a presidential election in late March. Its relations with Russia have been very poor since Moscow annexed the Crimea peninsula in 2014 and started backing armed separatists in eastern Ukraine.

SBU deputy head Viktor Kononenko told a news briefing that a group of Russian citizens and their Ukrainian collaborators had used financial bribes to set up a network of people ready to vote for a certain candidate and to influence public opinion.

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"This activity is illegal and implies an impact on the election results," Kononenko said, adding that the plot involved "citizens of Ukraine who have been cooperating with Russian structures for a long time".

Kononenko declined to say which of the 44 registered candidates stood to benefit from the scheme.

The incumbent, the pro-Western Petro Poroshenko, veteran opposition leader Yulia Tymoshenko and comic actor Volodymyr Zelenskiy are the frontrunners in the election race.

The latest opinion poll puts Zelenskiy in first place, with Poroshenko second and Tymoshenko third.

Russian authorities have denied any interference in Ukraine's election campaign.

Kiev has also previously accused Moscow of orchestrating large-scale cyber attacks as part of a "hybrid war" against Ukraine, which Russia also denies.

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Kononenko said the organizers of the vote-buying scheme hoped that 680,000 people would vote for a certain candidate as the result.

"This pyramid involved citizens of Russia, citizens of other countries and citizens of Ukraine who participated in the work of the political headquarters on the territory of Russia. We are working exclusively on the Russian traces," he said.

The head of Ukraine's cyber police, Serhiy Demedyuk, told Reuters last month that hackers probably controlled by Russia were stepping up efforts to disrupt Ukraine's election with cyber attacks on electoral servers and personal computers of election staff.