Turkey's electoral body has ordered that Istanbul's local elections be re-held after a shock opposition victory in March.
President Recep Tayyip Erdogan's AK Party questioned the slim win by the opposition CHP, claiming there were "irregularities and corruption".
CHP deputy chair Onursal Adiguzel said the rerun showed it was "illegal to win against the AK Party".
The vote will reportedly be held in the city on 23 June.
An AKP representative on the electoral board, Recep Ozel, said the rerun was because some electoral officials were not civil servants and some result papers had not been signed.
Mr Adiguzel tweeted that the decision was "plain dictatorship".
"This system that overrules the will of the people and disregards the law is neither democratic, nor legitimate," he wrote.
CHP candidate Ekrem Imamoglu was officially confirmed Istanbul's mayor by the authorities in April.
A supporters' group for Mr Imamoglu urged restraint, saying: "Let's stand together, let's be calm... We will win, we will win again."
Municipal elections took place across Turkey on 31 March and were seen as a referendum on Mr Erdogan's leadership amid a sharp economic downturn.
Although an AKP Party-led alliance won 51% of the vote nationwide, the secularist CHP claimed victory in the capital Ankara, Izmir, and in Istanbul - where Mr Erdogan had once been mayor.
The ruling party has since challenged the results in Ankara and Istanbul, which has prompted opposition accusations that they are trying to steal the election.