Ukrainians will cast their votes in a landmark presidential election on Sunday, months after prolonged popular protests drove former president Viktor Yanukovych out of office, setting off a violent separatist movement in the east and inviting an angry response from Russia.

The election takes place amid simmering tensions in Donetsk, Luhansk and other eastern regions, where dozens were killed in violent clashes two weeks ago.

Ukraine's national election watchdog said it expects voting in 95% of the country's polling stations to take place smoothly. However, it fears the process will be seriously disrupted in at least half the stations in rebel-controlled Luhansk and Donetsk.

Businessman Petro Poroshenko, who was in the forefront of the anti-Russia protests that overthrew Yanukovych, is widely expected to win while former prime minister Yulia Tymoshenko is placed second in opinion polls.

Security has been boosted across the country and the Kiev authorities expect angry reprisals by pro-Moscow separatists in the eastern region.

As many as 1,000 international observers from the Organization for Security and Cooperation in Europe (OSCE) are being deployed at polling stations across the country.

However, observers deployed in Donetsk have returned, citing "a campaign of terror" unleashed by pro-Russian separatists against Ukrainian electoral officials, Reuters reported.

More than 75,000 security personnel are on duty across the country, which has a population of 45 million, on poling day.

Voting starts at 8am (0500 GMT) and the polls will be open for the next 12 hours. The trends will be known by 8pm when exit poll results are released. The international observers will release the results on Monday.

Front-runner Poroshenko, who is known as the "chocolate king", will need to secure more than 50% of the votes to secure an outright victory.

If the 48-year-old confectionery tycoon does not get past that mark, a run-off election will be held on 15 June, in which his most likely opponent will be Tymoshenko.

The West has offered full backing to Kiev, expecting the election of a new president will help resolve the confrontation with Moscow.

Putin's positive signal

In a positive sign, Russian President Vladimir Putin said he would respect the outcome of the elections.

Ukraine, the second most populous ex-Soviet state, was wracked by anti-Russia protests demanding the removal of pro-Russia president Yanukovych.

But his eventual ouster opened up festering wounds in the east and triggered a violent separatist movement.