The six were part of a group of more than 300 left-wingers in about 30 jails who have pledged to fast to the death in a fight to achieve the closure of Eskisehir prison - dubbed the Coffin by inmates - and protest against prison conditions in general.
Dozens of hunger-strikers were reported by lawyers to be in a critical condition. So far, there has been little talkof force-feeding the inmates - partly out of fear of sparking widespread prison unrest.
Human rights workers said Mujdat Yanat, 37, died in Turkey's western Aydin prison, where he was serving an 18-year sentence for armed left- wing activities.
Earlier, Ali Ayata, an inmate, died in Bursa prison, and Huseyin Demircioglu starved himself to death in Ankara central prison, according to lawyers and rights activists. Three other hunger- strikers died earlier this week.
The prisoners and their lawyers also accuse prison officials of dispersing left-wing prisoners across the country, depriving them of family visits and also legal counsel.
Justice Minister Sevket Kazan, criticised by prisoners' families for an apparent lack of will to stop the protest, promised that jails would soon be back to normal.
"We will bring peace to the prisons as soon as possible," he said at a municipal ceremony in Istanbul. He did not say what measures would be taken.
"The Government's Disgrace," said secularist daily, Milliyet on its front page, highlighting the risks for Turkey's new Islamist-led coalition.
Mr Kazan, of the Islam-based Welfare Party, has so far refused to close Eskisehir, saying conditions there exceeded Western standards, and arguing that its individual cells kept the leftist prisoners properly under control.
Most Turkish prisons rely on large open wards, which, officials say, are difficult to monitor.Reuse content