Some see a personal element to Erdogan's grudge. Once mayor of Istanbul himself, the CHP has now won three consecutive mayoral elections against Erdogan's party in Turkey's biggest city, each by a greater margin than the last.

"We are up against huge bullying," Imamoglu said after his initial arrest earlier this week. "But I will not back down."

Erdogan extended his rule into a third decade after winning 2023's knife-edge presidential election. The next vote is not for three years, but some analysts say Erdogan could call for early elections that would allow him to bypass term limits.

Murat Somer, politics professor at Ozyegin University in Istanbul, said Imamoglu's detention showed Turkey was undergoing a political transformation from "an open autocratic regime to a Russian- or Belarussian-style, fully authoritarian, autocratic regime."