By Marta Maciag, Canan Sevgili and Julia Kotowska

(Reuters) - Poland will hold the first round of a presidential election on May 18, setting the scene for a vote that will be crucial for the pro-European government's hopes of implementing its reformist agenda.

The election pits liberal Warsaw Mayor Rafal Trzaskowski, a senior member of the ruling Civic Platform party (PO), against historian Karol Nawrocki, who runs Poland's Institute of National Remembrance, and far-right Slawomir Mentzen.

Here is a guide to the election, including candidate profiles and key issues:

WHAT'S AT STAKE?

Prime Minister Donald Tusk's coalition came to power in 2023, vowing to undo the previous Law and Justice (PiS) government's judicial reforms that critics said eroded the rule of law.

However, its efforts have been hampered by former nationalist PiS government ally President Andrzej Duda, who has the power to veto legislation.

HOW DOES THE ELECTION WORK?

Voters will select the president for a five-year term through a two-round system. If no candidate scores more than 50% in the first round, a run-off will be held between the top two contenders on June 1.

Presidents can serve a maximum of two terms. Duda's second term ends on August 6.

Polls open at 7 a.m. (0500 GMT) and close at 9 p.m. on May 18. Around 29 million people are eligible to vote.

Exit polls will be published shortly after voting ends. Partial results will trickle in throughout the evening on Sunday and on Monday.

WHO ARE CANDIDATES?

Rafal Trzaskowski

The frontrunner, 53-year-old Trzaskowski has served as the mayor of Warsaw since 2018. He was narrowly defeated by Duda in the 2020 presidential vote.

Trzaskowski's main proposals include increasing defence spending to 5% of GDP, liberalising abortion laws and developing Poland's arms and technology industry, as well as ensuring Poland has a strong position in the European Union.

Karol Nawrocki

A conservative historian, 42, Nawrocki is backed by the nationalist Law and Justice (PiS) party, which lost power to Tusk in 2023.

Between 2017-21, he ran the Museum of the Second World War in Gdansk, in northern Poland.

He wants to see lower taxes, a withdrawal from the European Union's Migration Pact and Green Deal, and is critical of giving more rights to LGBT couples.

Like Trzaskowski, he wants to allocate 5% of GDP to defence.

He is currently polling in second place.

Slawomir Mentzen

Far-right candidate Slawomir Mentzen, 38, runs a tax advisory firm and is a vocal critic of government regulation.

His key proposals include significant tax cuts, withdrawing from the European Green Deal and opposing more gay rights.

He believes Poland should avoid taking on excessive military or financial responsibilities in the Ukraine-Russia war. He wants to prioritise Poland's sovereignty by ensuring its constitution overrides EU laws.

He is currently polling in third place.

Szymon Holownia

The speaker of Poland's lower house of parliament, 48-year-old Holownia previously worked as a journalist and television presenter. He founded the political movement Polska 2050, a centrist grouping that is now part of the governing coalition.

Holownia's key proposals include promoting regional development, improving public transport and increasing access to affordable housing. He wants to reduce bureaucracy, support Polish businesses, and develop Poland's domestic arms production capabilities.

The left

Three leftist candidates are also in the race, with 43-year-old Magdalena Biejat, Deputy Senate Speaker, having the highest support. She is one of the most vocal proponents of women's and minority rights, and wants to allow access to abortion after a near-total ban on the procedure was introduced under PiS.

Biejat's proposals focus on reducing poverty, increasing public sector wages and making housing more affordable.

Her programme is similar to that of 45-year-old Adrian Zandberg, head of the Razem (Together) party.

The last leftist candidate is 76-year-old Joanna Senyszyn, an academic, lawmaker and former member of the Polish United Workers' Party which ran the country until the fall of Communism.

Other candidates

Also running in a 13-candidate field is the far-right's Grzegorz Braun, who in 2023 used a fire extinguisher to put out Hanukkah candles in the country's parliament, an incident that caused international outrage.

Another candidate is 42-year-old Krzysztof Stanowski, a journalists and YouTuber, who has no political programme and says he wants to show people the campaign behind the scenes and raise money for charities.

(Reporting by Marta Maciag, Canan Sevgili and Julia Kotowska)