German Chancellor Olaf Scholz's Social Democrats (SPD) on Sunday suffered a heavy setback in a regional poll in Berlin, falling behind the opposition conservatives in the capital for the first time in over two decades.

In a court-ordered rerun of the election in 2021, the Chancellor's party slipped to their worst postwar result in the capital, 18.2%, where they have held the mayor's office since 2001.

The conservative Christian Democratic Union of Germany (CDU) surged to around 28%, up from 18%, an exit poll by broadcaster ZDF showed.

Berlin, one of the country's 16 federal states, was ordered by courts to return to the ballot boxes after the 2021 election was found not to meet basic democratic standards amid widespread voting issues.

The outcome of the rerun puts a question mark on the continuation of the coalition between the Social Democrats, Greens, and far-left Linke party.

Early figures suggested that the Greens could finish ahead of the SPD with around 18.2% of the vote. The Linke party's share of the vote was meanwhile estimated at around 12.8%.

The Social Democrats seem to have been penalized by voters following violent scenes in the capital over the New Year, with revelers targeting the emergency services with fireworks in neighborhoods with a large immigrant population.

The election was also marked by discontent over rising rental prices in the capital and disputes over transport policy.

The result of the elections reflects the Social Democrats' struggles nationally, as Scholz's government -- in power for just over a year -- wrestles with rampant inflation and the fallout of the war in Ukraine.

The difficulties for Scholz's coalition with the Greens and the liberal Free Democratic Party (FDP) have been to the benefit of opposition parties. Along with the conservative CDU, the far-right Alternative for Germany (AfD) increased its share of the vote slightly to around 9% in Sunday's rerun, according to the estimates. 

The rerun in Berlin is only the second time in German postwar history that a state election has been declared invalid. In 1991, irregularities were reported in a vote in Hamburg.

The decision to organize the 2021 election on the same day as a national vote, a local housing referendum, and the Berlin marathon led to widespread logistical problems. 

Ballot papers got stuck in traffic as roads were closed for the marathon, with lines forming outside polling stations struggling to process votes.

Sunday's election took place under the watchful gaze of international election observers from the Council of Europe, invited in by the city itself to restore trust after the 2021 fiasco.

Berlin also called up 42,000 election helpers -- 8,000 more than last time around -- and gave polling stations extra ballots to avoid shortages.

The German parliament has also resolved for national elections to be partly repeated in Berlin, with the date of that vote yet to be set.

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