Imamoglu, who is generally seen as President Recep Tayyip Erdogan's main challenger, was arrested for alleged corruption on March 19. Since then, thousands of people have participated in almost daily street protests, despite the ban on demonstrations.
On Saturday, demonstrators on ferries crossing the Bosphorus to the Asian side of the city chanted: "Everywhere is Taksim, resistance is everywhere!"
The slogans refer to the famous Taksim Square in the city, which was the center of large protests in 2013. The President's regime has banned demonstrations there for several years, citing security reasons.
Opposition politician Özgur Özel told French Le Monde that he plans to make Saturday's protests a recurring feature every week.
The protests against Imamoglu's arrest spread quickly across Turkey and have several times escalated into clashes with police. Thousands of people have been arrested.
The police kept their distance during Saturday's demonstration and no new arrests have been reported.
The authorities have also cracked down on media coverage and arrested both Turkish and foreign journalists, including Swedish journalist Joakim Medin, who had traveled there to cover the protests.
