The 17 prisoners, including reformist journalists Bahman Ahmadi Amoui and Keyvan Samimi, as well as student activists Abdollah Momeni and Majid Tavakoli, were arrested for their part in a wave of protests that followed the disputed June 2009 vote.
Demonstrations erupted after President Mahmoud Ahmadinejad was reelected amid allegations of widespread fraud.
At least 10 of the hunger strikers received death sentences on charges of harming national security and provoking unrest.
According to the website of opposition leader Mir Hossein Mousavi, five of those participating in the strike have been hospitalised and the condition of others is deteriorating.
Another opposition website Rahesabz.net says that the hunger strike is also a response to an argument between some prisoners and wardens concerning the treatment of relatives visiting on 26 July.
The families of the protesters are expected to announce their intention to join the hunger strike in solidarity with their relatives.
Several prisoners held in connection with the protests have initiated a number of hunger strikes over the past year. Acclaimed film director Jafar Panahi was released on bail in May after three months in detention and several days of refusing to eat.