Imamoglu, Turkish President Tayyip Erdogan's main political rival, was taken in on Wednesday facing charges of graft and aiding a terrorist group, a move that the opposition condemned as a "coup attempt" and that sparked an initial round of demonstrations.
The move against the popular two-term mayor caps a months-long legal crackdown on opposition figures across the country which has been criticised as a politicised attempt to hurt their electoral prospects and silence dissent.
"We must stand against this evil as a nation," Imamoglu said on social media platform X, calling on members of the judiciary and Erdogan's ruling party to fight injustice.
"These events have gone beyond our parties, political ideals. The process is now concerning our people, namely your families. It is time to raise our voices," he said.
The government denies the accusations and has warned against tying Erdogan or politics to Imamoglu's arrest, after which it imposed a four-day ban on gatherings and restricted access to some social media to restrict communications.
Police on Thursday blocked off roads and stationed trucks with water cannons near the police station where the mayor is held and other areas of Turkey's largest city.
"They hastily detained our mayor, whom we elected with our votes," said Ali Izar, an opposition supporter on his way to work in central Istanbul.
"I do not think this is a democratic practice and I condemn it."
Although civil disobedience has been dramatically curbed in Turkey since the country-wide Gezi Park protests against Erdogan's government in 2013, which prompted a violent state crackdown, thousands of protesters took to the streets and university campuses on Wednesday in Istanbul, Ankara and other cities.
Crowds had chanted anti-government slogans and, at the main municipality building in Istanbul, they hung banners of Imamoglu and the country's founding leader Mustafa Kemal Ataturk alongside Turkish flags.
The detention of Imamoglu, 54, who leads Erdogan in some opinion polls, came days before his Republican People's Party (CHP) was set to elect him its next presidential candidate.
"The people will give their response again," said another Imamoglu supporter, Yusuf Demirci, 34, in Istanbul.
"As you saw yesterday, everyone is on the streets and in the squares. The squares and the ballot box will be the result of this. I say, the righteous one will win."
The detention sparked a crash and partial recovery in the lira currency on Wednesday, which by Thursday was worth 38 to the US dollar, compared to 36.67 beforehand.
Amid worries about eroding rule of law and concerns over slower rate cuts, bank shares in particular tumbled on the Istanbul bourse.
Interior Minister Ali Yerlikaya said that 37 people had been detained and accused of "provocative social media posts inciting crime and hatred" after the detention, adding 261 social media accounts, including 62 based abroad, were also identified.
Authorities also seized a construction company co-owned by Imamoglu - Imamoglu Construction, Trade and Industry - and handed control over to a court, according to the Istanbul Chief Public Prosecutor's office and financial crime investigation reports.
Istanbul municipality staff made up the bulk of the 105 people arrested along with Imamoglu.
