(Reuters) - Ukrainians voted on Sunday in an election that is likely to install a pro-Western parliament and strengthen President Petro Poroshenko's mandate to end separatist conflict in the east, but may fuel tension with Russia.

Voting started on a cold but sunny morning in Kiev in the first parliamentary election since street protests n the capital last winter forced Moscow-backed leader Viktor Yanukovich to flee and ushered in a pro-Europe leadership under Poroshenko.

A loose political grouping that backs Poroshenko is expected to become the leading force in the 450-seat assembly, giving him a mandate to pursue a peace plan for the east and carry out deep reforms sought by Ukraine's European Union partners.

Poroshenko said in a televised address on Saturday he wanted a majority to emerge that would see through laws to support a pro-Europe agenda and break with the Soviet past.

"Without such a majority in parliament, the President's program ... will simply remain on paper," he said.

With diminished pro-Russian influence and following a strong European integration agenda, it will be one of the most radical parliaments since Ukraine gained independence in 1991.

The emergence of a strong force committed to a united Ukraine could place new strains on ties with Russia which the Kiev leadership blames for backing rebels in a conflict that has killed more than 3,700 people and destroyed the economy.

There were no immediate reports of heavy clashes during the night in the east, where a fragile ceasefire has been in force since Sept. 5.

A gas pricing row with Russia which has the potential to disrupt supplies to European Union countries via Ukraine also rumbles on unresolved despite a meeting between Poroshenko and Russian President Vladimir Putin. 

Poroshenko called the snap election with the aim of clearing out Yanukovich loyalists and securing further legitimacy for Kiev's pro-Western direction after the "Euromaidan" protests.

The protests were broadly supported by the West but denounced by Russia as a coup after Yanukovich's fall. A month later, Russia annexed Crimea and separatist rebellions, supported by Russia, erupted in the industrialised east.

The ensuing crisis, in which the United States and its Western allies have imposed sanctions, is the worst between Russia and the West since the end of the Cold War.


Virtually all the leading parties have been campaigning on the need to fight corruption and ending the conflict in the east while keeping the eastern territories within a united Ukraine.

After a warning by Prime Minister Arseny Yatseniuk of possible "terrorist" attacks, more than 60,000 police were drafted in to guard polling stations, candidates and party headquarters across the country of 46 million before the conflict, in which Russia has annexed the Crimea peninsula.

In all, 29 parties are running, though only a handful are expected to reach the 5 percent barrier required to secure representation in parliament.

Many parties have enlisted war veterans and "Euromaidan" activists as candidates - which will add to the strong patriotic and nationalist tone of the new parliament.

Polling stations will open at 8 a.m. and close at 8 p.m. with exit polls available almost immediately.

About 2,000 international observers, including a team of about 800 from the Organization for Security and Cooperation in Europe, are in place to monitor polling procedures. 

Many voters will not be able to vote in Crimea and parts of the east where separatists are in control. Election authorities said voting would not take place in 27 constituencies, including 12 in Crimea, meaning that only 423 deputies will be elected. .

The separatists themselves, entrenched in the big industrial cities of Donetsk and Luhansk, are ignoring the election and say they will hold a rival poll on Nov. 2. Poroshenko and Western governments have denounced the planned poll as illegitimate.

Though opinion polls have put Poroshenko's bloc - comprising his Solidarity party and the Udar party of former boxing champion Vitaly Klitschko - well out in front, he may not win an outright majority.

But he should have no difficulty in putting together a coalition with other partners, such as Yatseniuk's People's Front, since virtually all the leading parties are pro-European, anti-Russian and favor a united Ukraine.

(Editing by Timothy Heritage)