KIEV (Reuters) - Ukraine's prime minister warned on Thursday of possible attempts by Russia to disrupt an election in Ukraine at the weekend, a vote being held against a background of Russian support for separatist rebels and an unresolved row over gas.

Sunday's poll is the first parliamentary election since street protests last winter drove Moscow-backed leader Victor Yanukovich from office and ushered in a pro-Western leadership.

The results are expected to turn a political bloc supporting President Petrol Poroshenko into the leading force in parliament, where pro-Russian influence will be greatly diminished.

Mr Poroshenko is seeking a mandate to press ahead with a plan for ending the conflict with separatists in Ukraine's Russian-speaking eastern regions and establishing an understanding with Moscow while pursuing a course of European integration.

Inter fax news agency quoted him as saying on Thursday that he expected to be able to begin forming a new coalition by early next week that would be "pro-European, anti-corruption, without liars and populists."

Western governments supported the "Euromaidan" winter protests in Kiev that forced Mr Yanukovich to flee to Russia, but Moscow denounced his overthrow as a coup.

Russia went on to annex Crimea and back separatists in a conflict that has killed more than 3,700 people.

With violence between government forces and separatists still simmering in eastern Ukraine despite a ceasefire, Prime Minister Arsenic Yatseniuk, a hawk in the Kiev leadership, ordered a full security mobilisation for the weekend to prevent"terrorist acts" being carried out.

"It is clear that attempts to destabilise the situation will continue and be provoked by the Russian side. They did not succeed during the presidential election (in May) ... but their plans have remained," he told a meeting of top security chiefs and election monitors.

"We need ... full mobilisation of the whole law-enforcement system to prevent violations of the election process and any attempts at terrorist acts during the elections," Mr Yatseniuk said.

There was no immediate reaction from Moscow to his charges. But Russian foreign ministry spokesman Alexander Lukashevich said Russia hoped the election would be held in accordance with"democratic principles and norms" and that "a process of gradual political stabilisation" of Ukraine would follow, RI news agency reported.

Mr Lukashevich said Russians would take part in monitoring the vote as part of an observer mission of the Organization for Security and Cooperation in Europe (OSCE).

"Realistically, we are in a state of Russian aggression and we have before us one more challenge - to hold parliamentary elections ... The choice (of voters) will be made by the ballot-paper and an honest expression of will and not automatic weapons," Mr Yatseniuk said.

An unresolved row over gas between Ukraine and Russia, its main energy supplier, is further testing relations and raising concern among European Union countries.

Many of them rely on Russian gas via Ukraine and worry the dispute could affect supplies to them this winter.

The two powers have agreed on a new price for Russian gas of$385 per thousand cubic metres. But they are still at odds over the volumes to be supplied and the level of Ukraine's debt for previous supplies, which Moscow puts at $4.6 billion.

The Soviet-era central heating system in Ukraine is largely gas-powered. With temperatures set to drop below freezing this weekend as the election takes place, Ukrainian authorities have switched on heating in apartment blocks 10 days ahead of time.

But as the conflict in eastern Ukraine disrupts coal supplies, shops have seen a run on sales of electric radiators as Ukrainians scramble to find alternative sources of warmth over winter.

In Kiev, one of the main botanical gardens announced it could no longer keep greenhouses heated and offered its stocks of tropical blooms and flora for sale.

This weekend, 61,000 police will be responsible for guarding polling stations across the country of 46 million, Interior Minister Arsen Avakov said.

Some 4,000 of them will be members of special forces who can react rapidly to any threat of"terrorist" action.

The annexation of Crimea means the loss of 12 seats from the 450-seat parliament, and separatist action in the east will prevent polling in at least 14 voting districts there.

Separatists say they plan to hold rival elections on Nov 2 in the territory they control to further their demands for independence.