Waving flags of the pro-Kurdish People's Democratic Party (HDP), they danced and flashed victory signs in Diyabakir, the main city in the region, to express solidarity with Leyla Guven.
The HDP describes Guven's condition as "life threatening."
The 55-year-old began a hunger strike on November 8 in protest at the prison conditions for Kurdish leader Abdullah Ocalan.
"It is our duty to add our voices to that of Leyla's," HDP's co-president Pervin Buldan told the gathering.
Ocalan is one of the founders of the outlawed Kurdistan Workers' Party (PKK) which has waged a bloody insurgency against the Turkish state since 1984, and which is blacklisted as a terror group by Ankara and its Western allies.
He has been serving a life sentence for treason in an island prison near Istanbul since his capture in 1999.
Guven's action is aimed at pressuring the government into allowing lawyers and family members to visit Ocalan, whose brother was finally allowed to meet him in jail a week ago.
She was arrested for her opposition to Turkey's military operation against a Syrian Kurdish militia that Ankara considers an offshoot of the PKK and has been in jail since January last year.
Agencies contributed to this report.
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