Alaa Abdel-Fattah was a major force in the 2011 revolt that toppled longtime autocrat Hosni Mubarak.
An outspoken blogger, Abdel-Fattah has been in and out of prison in the three years since Mubarak's ouster. He campaigned against military trials for civilians during the 17 months that generals held power following Mubarak's resignation.
Abdel-Fattah opposed Islamist President Mohammed Morsi as well, but strongly disapproved of the military's return to politics in the wake of Morsi's ouster.
In a statement today, the activist was quoted as saying: "I will no longer play the role they have written for me." It said that he started his hunger strike yesterday night. The family said it held authorities responsible for Abdel-Fattah's safety.
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His hunger strike comes a day after he visited his unconscious father, a longtime rights lawyer, in hospital after undergoing surgery.
"The scene of his unconscious father ... Was a turning point for Alaa and at the end of the visit he decided he will not cooperate with this ... Unjust situation even if the price is his life," the statement read.
The June sentence was the toughest against any of the secular activists behind the 18-day uprising that ended Mubarak's 29-year reign. It is also the first conviction of a prominent activist since former army chief Abdel-Fattah el-Sissi took office as president.