This was the first record of a fatal event in seven days of protests across the country.
Reports said Abdullah Cömert was demonstrating to show support for a wider protest that began in İstanbul last week against the policies of Prime Minister Recep Tayyip Erdoğan. Cömert was shot in the head, initial reports said.
Commenting on the issue, Deputy Prime Minister Bülent Arınç said on Tuesday that the reason why the protester was killed is being investigated.
Cömert was a member of the main opposition Republican People's Party's (CHP) youth branch, private NTV television reported, citing a CHP lawmaker from Hatay.