The government said it had arrested 121 people in March and accused them of being members of the outlawed Oromo Liberation Front (OLF) rebel group.
Campaign group Human Rights Watch said the arrests were part of a government crackdown on opposition campaigners from Ethiopia’s Oromo ethnic group.
Ethiopian Prime Minister Meles Zenawi is Washington’s closest ally in the Horn of Africa but has come under increasing criticism from rights groups accusing him of crushing dissent.
Opposition officials in the capital Addis Ababa who did not want to be named told Reuters they suspected the arrests were motivated by government fear of a public uprising.
Ethiopia’s government defended the arrests and said there was no limit to how long the suspects could be held as long as they were regularly remanded by the courts.
“They are being held on suspicion of OLF membership while prosecutors prepare charges against them for terrorist activities,” government spokesman Shimeles Kemal told Reuters.
“Ethiopia has the right to defend itself against terrorism and these groups have no right to question that … Nobody was arrested only for involvement in opposition politics,” he added.
Prime Minister Meles Zenawi told parliament on Tuesday that some “terrorists” were using membership of political parties to cover up their activities.
He said Eritrea was stepping up attempts to destabilise Ethiopia by arming rebel groups including the OLF.
The OLF has been fighting since 1993 for more autonomy for the Oromia region. The Oromo are Ethiopia’s biggest ethnic group with 27 million out of a population of 80 million.
Oromia produces most of the coffee in Africa’s biggest grower, along with oil seeds, sesame and livestock, which are all key exports.
Human Rights Watch said Ethiopia had arrested more than 200 Oromos since March.
“The authorities should immediately free the Oromo opposition members unless they can bring credible charges against them,” the New York-based group said in a statement.
Opposition parties said their supporters had been arrested in recent weeks.
“We have more than 80 people detained since mid-March,” Merera Gudina, leader of the Oromo People’s Congress, told Reuters. “The government, however, says they are members of the OLF. The arrests are ongoing.”
The main opposition coalition, Medrek, last week named 68 Oromos it said