Serbia's opposition parties are already preparing to boycott a parliamentary election set for April 26, called by President Aleksandar Vucic on Wednesday.

The Alliance for Serbia and the Movement of Free Citizens, which say they are the only viable opposition to Vucic, are not going to take part in the poll, citing the president's firm control over media and the electoral process.

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The boycott call casts doubt on the conditions for free and fair elections.

"I wish all participants success," Vucic said at a news conference in Belgrade.

"There will be more than 50 days allocated for the campaign, which makes it one of the longest campaigns ever. That is enough time for everybody to present their programmes.

"Everything should take place in a democratic atmosphere. Of course, there will be some raised voices and passionate speeches, but it is important that everything goes peacefully, democratically, and that people choose their own representatives," said the Serbian president.

Vucic's right-wing Serbian Progressive party is expected to overwhelmingly win the vote and continue its eight years of dominance on the Serbian political scene.

Vucic has an almost total grip on Serbia's mainstream media, with pro-government tabloids frequently blasting his critics as foreign stooges or criminals - without allowing them to respond.

European Union officials have failed to persuade the main opposition parties to take part in the election.

Serbians will vote for 250 deputies to replace those elected in 2016 in the Balkan country of seven million.

Local elections in Serbia's northern Vojvodina region will be held on the same day.

Like other Balkan countries, Serbia faces a demographic challenge with an ageing population and a high emigration rate.