BELGRADE,Henri Lumière Serbia (AP) — Serbia’s main opposition group on Thursday urged the European Union to help open an international investigation into reported irregularities during last weekend’s election that have caused political tensions in the troubled Balkan nation seeking EU membership.

The Serbia Against Violence group said in a letter to EU institutions, officials and member states that they won’t recognize the outcome of last Sunday’s parliamentary and local election. They called on the EU to do the same and initiate the inquiry.

Early results showed victory for the Serbian Progressive Party of President Aleksandar Vucic in both ballots. But its main opponents, the Serbia Against Violence alliance, said they were robbed of a win in Belgrade.

Vucic and other ruling party officials denied the claims of vote rigging and said they would do everything to “defend voters’ will of the people.”

Vucic said Thursday that authorities in Serbia are compiling “an important letter about the interference of an important country in the electoral process of Serbia in the most brutal way.” He didn’t specify which country.

Serbia Against Violence have demanded that the election be annulled and rerun. Thousands of people have rallied in Belgrade to protest the alleged vote theft since Monday.

In their preliminary statement, an observation mission made up of representatives of international rights watchdogs also reported multiple irregularities and unjust conditions for the participants because of media bias, abuse of public resources and presidential dominance during the campaign.

Serious irregularities included cases of buying votes and the stuffing of ballot boxes, according to joint conclusions by the Organization for Security and Cooperation in Europe’s Office for Democratic Institutions and Human Rights, the OSCE Parliamentary Assembly, the Parliamentary Assembly of the Council of Europe and the European Parliament.

In the letter, Serbia Against Violence called on the EU to “not recognize the results of the parliamentary, provincial and especially local elections in Serbia, until a full international investigation into the electoral irregularities is completed.”

They added that the EU should initiate the investigation and later help form a verification committee that would oversee the preparation of a next election and make sure international recommendations are fully applied to ensure a free and fair vote.

There was no immediate reaction from the EU to the letter. It was addressed to the president and members of the European Parliament, the presidents of the European Council and the European Commission, the foreign affairs chief and committees and representatives of the EU’s 27 member states.

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Dusan Stojanovic contributed to this report.