Early in the morning on March 19, dozens of armed police officers showed up at my door with a detention order. The scene resembled the capture of a terrorist, not of the elected mayor of Istanbul, Turkey’s largest city.

The move — four days before my party, the Republican People’s Party, was to hold a primary for the next presidential race — was dramatic but hardly unexpected. It followed months of escalating legal harassment of me, culminating in the abrupt revocation of my university diploma 31 years after I had graduated. Authorities seemed to believe this would disqualify me from the race because the constitution requires the president to have a degree in higher education.

Realizing he cannot defeat me at the ballot box, President Recep Tayyip Erdogan has resorted to other means: having his main political opponent arrested on charges of corruption, bribery, leading a criminal network and aiding the outlawed Kurdistan Workers’ Party, even though the charges lack credible evidence. I was suspended from my elected office over the financial charges.

For years, Mr. Erdogan’s regime has gnawed away at democratic checks and balances — silencing the media, replacing elected mayors with bureaucrats, sidelining the legislature, controlling the judiciary and manipulating elections. The large-scale arrests of protesters and journalists in recent months have sent a chilling message: No one is safe. Votes can be nullified and freedoms can be stripped away in an instant. Under Mr. Erdogan, the republic has been transformed into a republic of fear.

This is more than the slow erosion of democracy. It is the deliberate dismantling of our republic’s institutional foundations. My detention marked a new phase in Turkey’s slide into authoritarianism and the use of arbitrary power. A country with a long democratic tradition now faces the serious risk of passing the point of no return.

The crackdown extended beyond me. In a sweeping operation built on an indictment that is no more than a compilation of statements by secret witnesses, the police detained nearly 100 people, including senior municipal administrators and business figures. Disinformation and defamation campaigns in pro-government media preceded the detentions.