Human rights groups have condemned the conviction of 24 Ethiopians, including a prominent journalist, on charges of conspiring with rebels to overthrow the government.

The case, the third terrorism verdict against journalists in six months, underlined concerns of growing repression under Meles Zenawi, the prime minister.

The Committee to Protect Journalists (CPJ) said Ethiopia was now close to overtaking its neighbour Eritrea as the African country with the most journalists in jail.

The journalist and blogger Eskinder Nega and two leading opposition politicians, along with five other men, were found guilty of "terrorist acts", "encouragement of terrorism", "high treason" and several other charges.

Judge Endeshaw Adane ruled in court on Wednesday that the defendants' main aim had been to spark an Arab spring-style revolt in the country. "Under the guise of freedom of speech and gathering, the suspects attempted to incite violence and overthrow the constitutional order," he said.

A further 16 men were found guilty in absentia, including several journalists and one human rights activist.

Nega, recently honoured with the prestigious Pen America press freedom award, was arrested last year and accused of trying to incite violence with a series of online articles. He was also accused of belonging to Ginbot 7, a political movement branded a terrorist organisation by the Ethiopian government.

Sentencing is expected on 13 July. Prosecutors said they would not demand the death penalty but have called for terms of imprisonment from five years to life for the group.

Rights activists accused the government of using national security concerns as an excuse to clamp down on opposition figures and journalists.

"This is a dark day for justice in Ethiopia, where freedom of expression is being systematically destroyed by a government targeting any dissenting voice," said Claire Beston, Amnesty International's Ethiopia researcher.

Amnesty claimed the trial was marred by serious irregularities, including allegations of torture made by at least one defendant, which were not investigated.

Human Rights Watch also criticised the verdict. Leslie Lefkow, its deputy Africa director, said: "This case shows that Ethiopia's government will not tolerate even the mildest criticism. The use of draconian laws and trumped-up charges to crack down on free speech and peaceful dissent makes a mockery of the rule of law."

Two members of the media were each jailed for 14 years on similar charges in February, two months after two Swedish journalists were imprisoned for 11 years for entering the country illegally and aiding a rebel group.

Critics point to an anti-terrorism law passed after several explosions in 2009 that states that anyone caught publishing information that could induce readers into acts of terrorism could be jailed for between 10 and 20 years.

More than 10 journalists have been charged under the law, according to the CPJ. "The Ethiopian government has once again succeeded in misusing the law to silence critical and independent reporting," said its Africa advocacy co-ordinator Mohamed Keita.

"The charges against Eskinder are baseless and politically motivated in reprisal for his writings. His conviction reiterates that Ethiopia will not hesitate to punish a probing press by imprisoning journalists or pushing them into exile."