Turkish prosecutors have launched 32 investigations into allegations of irregularities made by the ruling party after its shock loss in Istanbul's local elections held on March 31.
The investigators summoned more than 100 polling station officials for questioning as suspects, state-owned Anadolu news agency reported on Thursday.
The investigations focused on Istanbul's Maltepe, Kadikoy and Atasehir districts, Anadolu said.
The suspects were being probed over allegations of violations of election law and abuse of position in counting votes and entering dates, it said.
The Istanbul prosecutor's office declined to comment.
President Recep Tayyip Erdogan's ruling AK Party has called for the annulment and rerun of the Istanbul vote, a prospect that has kept Turkish financial markets on edge.
Initial results and some recounts of the nationwide local elections showed that AK Party lost control of Istanbul and the capital, Ankara.
In a blow to Erdogan, the main opposition secularist Republican People's Party (CHP), in alliance with smaller parties, won the mayoralty in both cities and ended a quarter-century of control by the AK Party and its Islamist predecessors.
CHP's Ekrem Imamoglu was declared Istanbul's new mayor last month, despite a pending appeal from the AK Party to invalidate the vote.
The High Election Board is yet to rule on the AK Party's challenge to the Istanbul election result. The electoral council is due to convene on Monday to discuss the appeal.
Since the election, votes have been recounted in dozens of districts, multiple times in some of them.
If the AK Party's request for a rerun is approved by the Istanbul election board, which is legally still possible, the vote could be repeated on June 2.
A month after the polls, AK Party officials are questioning their alliance with the smaller Nationalist Movement Party, which some blame for one of its biggest electoral setbacks.