Twenty Ethiopians, including a prominent blogger, journalists and opposition figures were jailed for between eight years to life on Friday on charges of conspiring with rebels to topple the government.
The European Union's foreign policy chief Catherine Ashton said she was "seriously concerned" by the severity of the sentences, which were also condemned by Amnesty International and other rights groups.
The Horn of Africa nation, a major recipient of Western aid, is fighting separatist rebel movements and armed groups it says are backed by arch-foe Eritrea.
But rights groups say Ethiopia, sandwiched between volatile Somalia and Sudan, regularly uses security concerns as an excuse to crack down on dissent and media freedoms.
The Addis Ababa government, a key ally in Washington's campaign against Islamist militants in Somalia, denies the charge. No one was immediately available to comment on Friday.
Blogger and journalist Eskinder Nega, who was arrested last year and accused of trying to incite violence with a series of online articles, was jailed for 18 years.
Five other exiled journalists were sentenced in absentia to between 15 years to life.
Andualem Arage, from the opposition Unity for Democracy and Justice (UDJ) party, was jailed for life. Two other prominent opposition figures, Berhanu Nega and Andargachew Tsige, both out of the country, also received life sentences.
"The court has given due considerations to the charges and the sentences are appropriate," Judge Endeshaw Adane said in court.
The 20 were charged last year, most of them in absentia, with counts including conspiracy to dismantle the constitutional order, recruitment and training for terror acts and aiding Eritrea and a rebel group to disrupt security.
They were also accused of belonging to Ginbot 7, a group branded a "terrorist" organization by the government.
Eskinder Nega's lawyer Abebe Guta told Reuters his client would appeal against the sentence. Nega wrote for a number of diaspora-based blogs.
"We can't even express our fundamental rights anymore," UDJ member Temesgen Zewdie told Reuters after the sentencing.
Critics point to an anti-terrorism law passed after several explosions in 2009 which says anyone caught publishing information that could incite readers to commit acts of terrorism could be jailed for up to 20 years.
More than 10 journalists have been charged under the law, according to the Committee to Protest Journalists. The group says Ethiopia is close to replacing Eritrea as the African country with the highest number of journalists behind bars.
Two journalists were each jailed for 14 years on similar charges in February, two months after two Swedish newsmen were sent to prison for 11 years on charges of entering the country illegally and aiding a rebel group.
The EU's Ashton was concerned about the sentences and that "a lack of clarity with regard to what constitutes a terrorism offence ... can affect the freedom of expression ... enshrined in the Ethiopian Constitution," a statement from her office said.
Amnesty International said: "The Ethiopian government is treating calls for peaceful protest as a terrorist act and is outlawing the legitimate activity of journalists and opposition members."
(Additional reporting by Robin Emmott in Brussels; Editing by Duncan Miriri and Andrew Heavens)