İmamoğlu, widely seen as the main political challenger to President Recep Tayyip Erdogan's 22-year rule, was jailed on 23 March pending trial on corruption charges.
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Hundreds of thousands of protesters have congregated in Istanbul to how their support for the city's imprisoned mayor and demand his release.
Turkey's main opposition party, the Republican People's Party (CHP) organised the demonstration, the latest in a series of protests that have resulted in hundreds of arrests and have dialled up the pressure on the country's long-time leader, President Recep Tayyip Erdoğan.
Istanbul Mayor Ekrem İmamoğlu, a key political rival to Erdoğan, was detained on 19 March on corruption and terrorism charges that many saw as politically motivated.
The government insists the judiciary is independent and free of political interference.
His detention, and later formal arrest on corruption charges on 23 March, sparked nationwide protests despite assembly bans, police crackdowns and legal prosecution by authorities.
"They've detained hundreds of our children, thousands of our youths... arrested hundreds of them," CHP leader Özgür Özel told protesters.
"They only had one goal in mind: to intimidate them, terrify them, make sure they never go out again."
Interior Minister Ali Yerlikaya said on Thursday that nearly 1,900 people had been detained since 19 March and pro-government media reported on Friday that public prosecutors had requested up to three years imprisonment for 74 of the detainees.
Police kept their distance at Saturday's rally with no new arrests reported.
Özel called for the immediate release of İmamoğlu, as well as for other political prisoners including Selahattin Demirtaş, a former presidential candidate and founder of the pro-Kurdish Peoples' Equality and Democracy Party (DEM).
"In the Turkey we envision, presidential candidates will not be imprisoned," added Özel.
Last Sunday, hours after he had been formally arrested, İmamoğlu won a symbolic primary to be the CHP's candidate in a presidential election currently scheduled for 2028, but which is likely to take place earlier.
Özel noted they would begin collecting signatures for İmamoğlu's release and also to demand an early election.