By Maria Tsvetkova and Pavel Polityuk

KIEV (Reuters) - President Volodymyr Zelenskiy looked set on Monday to become Ukraine's first leader since the fall of communism to command a single party majority in parliament, in what would be an unprecedented mandate to deliver promised reforms.

His party's victory in Sunday's snap parliamentary election caps a meteoric rise for the former TV comedian who has tapped into widespread anger over entrenched corruption and low living standards in one of Europe's poorest countries.

Zelenskiy's Servant of the People party, named after the satirical TV series where he played a humble history teacher who accidentally becomes president, was on course to win 246-249 seats out of 424 seats, his party estimated.

That would give Zelenskiy, 41, the power to decide the make-up of the next government and control over a parliament that had previously been loyal to his predecessor Petro Poroshenko and blocked his legislative agenda.

Since defeating Poroshenko by a landslide in April's presidential race, Zelenskiy has promised to keep Ukraine on a pro-Western course and seek a new aid-for-reforms program with the International Monetary Fund.

He has also pledged to find a lasting peace in eastern Ukraine's Donbass region, where war between Kiev's forces and Russian-backed armed separatists has killed 13,000 in five years since Moscow's annexation of Ukraine's Crimea peninsula in 2014.

His back-to-back election victories "create the necessary support for the newly elected president to implement much-needed and long-delayed reforms," a note by Citi said.

Markets welcomed the prospect of a period of political stability, with some of Ukraine's dollar-denominated sovereign bonds rising to their highest since early 2018.

Russia's foreign ministry said the election result offered hope but added that Ukrainian lawmakers should use their mandate "wisely for peaceful purposes and for the benefit of the population of the whole country."

WEIGHT OF EXPECTATIONS

Under Ukraine's mixed election system, Zelenskiy's party won in both party list voting and in constituencies, preliminary election commission data showed.

It would be the first time since Ukraine won independence from the Soviet Union in 1991 that most members of parliament belong to one political party.

"Zelenskiy will have effective control over the choice of prime minister and government, making the presidency de facto the dominant Ukrainian political institution," Daragh McDowell of the risk consultancy Verisk Maplecroft said.

"While this will make it easier for Zelenskiy to push through reforms it also undermines the checks and balances of Ukraine's fragile political institutions."

Power is shared in Ukraine between the president and parliament.

The president nominates key appointments including the head of the central bank, the foreign and defense ministers and the head of the armed forces, while the biggest party or coalition in parliament determines the make-up of the government.

In Ukraine's fractious politics, where parliamentary disputes have sometimes ended in mass brawls, hostilities between the government and president have stymied policymaking in the past.

Zelenskiy will now have to manage the high expectations that his large mandate has generated.

A pre-election survey by the Washington-based National Democratic Institute said 45% of voters expected Zelenskiy to negotiate a peace in the Donbass within 12 months -- the biggest single priority among voters.

But 57% would not accept peace at the cost of allowing Crimea to become a recognized part of Russia -- something Moscow is likely to insist on -- and 62% would not accept peace if Donbass did not return to Kiev's full control.

More than half of respondents also expected Ukraine to be a member of the European Union by 2030.

"There should be changes in our country. I am 49 years old and judging from my experience, I know we had no other way but to make such a choice. Otherwise, we would not be able to get rid of the past," said Olena Moskalenko, a finance specialist from Kiev.

"I have one hope only - that we won't get disappointed in our choice again and in our expectations," she told Reuters TV.

Parliament nominally has 450 seats but in this election the number of constituencies fell to 199 from 225, reflecting the loss of control over Crimea and parts of the Donbass region.

The Russian-friendly Opposition Platform was in a distant second place, followed by Poroshenko's party and that of a former prime minister, Yulia Tymoshenko.

Another new pro-Western party, Voice, fronted by rock star Sviatoslav Vakarchuk, also passed the 5% threshold to enter parliament. Voice is the only party to which Zelenskiy has offered coalition talks.

Vakarchuk said it was too early to discuss a potential alliance while votes were still being counted.

(Reporting by Pavel Polityuk, Natalia Zinets, Matthias Williams and Bogdan Basii in Kiev and Karin Strohecker in London; writing by Maria Tsvetkova and Matthias Williams; Editing by Gareth Jones)