An Egyptian court has sentenced prominent dissident Hisham Kassem to six months in prison, his lawyer and political movement said, a move barring him from taking part in campaigning for next year's presidential election.

It comes one day after Egypt's only candidate campaigning so far for the election, Ahmed al-Tantawi, revealed his phone had been bugged by authorities, according to a report by the University of Toronto's Citizen Lab.

A day earlier, Tantawi had denounced harassment by the security forces against his teams and supporters.

"Kassem was also slapped with a 20,000 pound (about $650) fine after being found guilty of defaming a former minister and contempt of officials," Gameela Ismail, a member of his Free Current liberal opposition movement, wrote on X, formerly Twitter.

His lawyer Nasser Amin wrote on Facebook that the verdict would be appealed in a court hearing set to take place on October 7.

Kassem, 64, had begun a hunger strike, his supporters said earlier this month, after the opening of his trial, before ending it days ago.

He was initially summoned after a former minister complained he had shared online articles suggesting the minister had embezzled funds.

The opposition activist was later accused of "contempt" by officers during questioning at a police station. He has been in custody since August 20.

Kassem's Free Current coalition, formed in June by opposition parties, advocates economic liberalisation and calls for an end to the army's stranglehold on the Egyptian economy.

London-based Amnesty International on Thursday called on Egypt's authorities to "immediately release" Kassem, saying he had been "arbitrarily detained".

"The prosecution of Hisham Kassem for simply posting critical messages online is a signal that the Egyptian authorities' relentless campaign to silence peaceful critics and punish dissent... is continuing in full force," said Philip Luther, Amnesty's research and advocacy director in the region.

Several opposition figures, including renowned activist Ahmed Douma, received presidential pardons in past weeks in what many analysts saw as a bid to curry public and international favour ahead of next year's election.

The government also launched a "national dialogue" last year seeking to bring in Egypt's opposition, which has largely been decimated since general-turned-president Abdel Fattah al-Sisi seized the reins in 2013.

Egypt has thousands of political prisoners; human rights groups estimate.

But despite the release of nearly 1,000 in the past year, non-governmental organisations say almost three times as many have been detained over the same period.

According to Gameela Ismail, Kassem was long viewed as a threat due to his criticism of the military's role in the economy -- which has been in freefall for over a year and is likely to be a key bone of contention in the upcoming polls.

Despite the allegations of harassment, Tantawi has said he is "determined" to persist with his presidential race.

The Egyptian Initiative for Personal Rights has meanwhile reported that at least 35 members of his campaign have been arrested in less than three weeks.

Sisi is widely expected to announce his candidacy for next year's election, though he has not yet done so.

The former army chief was elected in 2014, a year after he led the military ouster of elected Islamist president Mohamed Morsi.

The United States recently approved most military assistance to Egypt despite persistent concern over human rights, the State Department said, stressing Cairo had been helpful in several hotspots.

New York-based Human Rights Watch on Friday said the move "disregards the Egyptian government's ongoing repressive policies".

"US officials are creating a false choice between national security and human rights," said Nicole Widdersheim, HRW's deputy Washington director.