A prominent Egyptian blogger has begun a hunger strike in protest at his detention and to raise awareness of what activists say is a widening crackdown on dissent by President Mohamed Morsy and his Islamist-led government.

Ahmed Douma was sentenced this week to six months in jail for calling Morsy a criminal and a murderer in media interviews. He has been in detention since his arrest in April.

After the sentencing he was meant to leave prison on bail pending an appeal, but was kept behind bars and charged again, this time with inciting violence.

Egypt's public prosecutor has since referred him and 11 others to trial in the case, the latest in a number of prosecutions of political activists.

"An example is being made of Douma," Douma's lawyer Ali Soliman told Reuters on Thursday. He said the blogger began refusing food on Wednesday.

The government denies that the growing number of cases against its critics are politically motivated, and Mursi has said he respects freedom of expression.

"Douma is an important revolutionary figure," said Malek Adly of the Cairo-based Arabic Network For Human Rights Information. "By targeting him, the prosecutor-general is carrying out the orders of the government which is trying to silence the opposition."

The spate of cases against activists comes ahead of a major protest planned by opposition groups on June 30, the date Morsy took office last year.

The organizers of "Tamarod" (Rebel) are gathering signatures for a petition calling for Morsy's removal and early elections.

They say they have collected more than seven million signatures and aim to collect 15 million - more than the 13 million votes Morsy received in last year's vote - before the protest.