The protesters gathered amid tight police security at a square in Bakirkoy on the European side of the city.
"I am saluting my friends resisting in prisons. They are our honour, they are not alone," said Fahit Ulas, a supporter of the pro-Kurdish Peoples' Democratic Party (HDP).
"If needed we will sacrifice our bodies for this cause. We have no fear," he insisted.
Around 250 prisoners all over Turkey have begun hunger strikes along with HDP deputy Leyla Guven, who wants to force the Turkish government to let Ocalan hold regular meetings with his lawyers and relatives, AP reported.
Ocalan, a founder of the PKK, a group blacklisted as a terrorist organisation by Ankara and Washington, has been sentenced to life in prison and has been held on a prison island near Istanbul since his capture in 1999.
Guven, 55, was jailed in January 2018 for criticising Turkey's military operation against a Syrian Kurdish militia.
Last week, a Turkish court in the predominantly Kurdish city of Diyarbakir freed her under judicial supervision. Guven, who marked the 88th day of her hunger strike on Sunday, launched her protest in jail on November 8.
She told AFP in an interview that she would press on with the hunger strike at home.
Garo Paylan, an HDP deputy who joined the Istanbul protest, said that staging a hunger strike was a "last resort," and told AFP: "We want this demand to be fulfilled before any death."
In 2012, hundreds of Kurdish prisoners ended a 68-day hunger strike after Ocalan urged them to do so.
Several HDP deputies have been jailed and in an interview Sunday with state-run TRT television, President Recep Tayyip Erdogan said: "The HDP is equal to the PKK ... It's crystal clear."
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