WARSAW—Protesters led by right-leaning journalists stormed the headquarters of Poland’s top electoral authority on Thursday evening, demanding local elections be held again because last Sunday’s vote led to discrepancies in the vote count.

Several dozen demonstrators called for the dismissal of electoral officials. One senior official on the electoral authority, Kazimierz Czaplicki, quit his post on Wednesday after the authority’s computers had been hacked and the vote-count system malfunctioned.

On Thursday evening, four days after voting stations had closed, the electoral committee’s website still only gave the official turnout information from Sunday afternoon. The authority has acknowledged its inability to provide full official results in a timely manner, blaming an outside technology contractor.

An exit poll by Ipsos for Poland’s three leading television stations on Sunday night showed the conservative opposition Law and Justice party in the lead in the election to regional legislative assemblies. The ruling Civic Platform party’s leader, Prime Minister Ewa Kopacz, quickly conceded defeat on Sunday night. Exit polls on election nights in the past have usually proved accurate.

But partial results given by the electoral committee on Tuesday raised eyebrows, indicating the Peasants Party, a junior governing coalition member in the national government, winning in 10 out of 16 regions in Poland.