A prominent Egyptian blogger has declared a hunger strike to protest his ongoing detention by authorities and raise awareness about what activists describe as a widening crackdown on dissent by President Mohamed Morsi and his Islamist-led government.

Ahmed Douma was sentenced this week to six months in jail for calling Morsi a "criminal" and "murderer" in televised comments. He has remained in detention since his arrest in April.

After the sentencing, he had been supposed to leave prison on bail pending appeal, but was kept behind bars on separate charges of inciting violence.

Egypt's prosecutor-general has since referred him – along with 11 others – to trial, the latest in a number of legal cases against political activists.

"An example is being made of Douma," Ali Soliman, Douma's lawyer, told Reuters on Thursday, adding that the blogger had begun refusing food on Wednesday.

The government denies that the growing number of cases against its critics are politically motivated, and Morsi 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 for 30 June – which will mark the end of Morsi's first year in office – by opposition groups.

The organisers of the Tamarod ('Rebel') campaign are gathering citizens' signatures for a petition calling for Morsi's removal and snap presidential elections.

They say they have gathered more than seven million signatures so far, and aim to collect a total of 15 million – more than the 13 million votes Morsi received in last year's presidential race – before the protest.