The government of Ethiopia said it had arrested 35 people it accused of plotting a coup planned by an Ethiopian-American university professor, Dr. Berhanu Nega.
Dr Berhanu was elected mayor of capital Addis Ababa after parliamentary elections in 2005 but was jailed when the opposition disputed the government's victory.
Police and soldiers killed about 200 opposition protesters in street violence that followed the poll.
Dr Berhanu and other opposition leaders were released in a 2007 pardon after which he went to the United States to teach economics at Bucknell University in Philadelphia.
"We raided their homes and got weapons, bombs, landmines, soldier uniforms and their future plans," government spokesman Ermias Legesse told the Reuters news agency.
Dr Berhanu's organisation 'May 15th' is named after the date of the 2005 poll. He has made statements in the United States saying it wants to violently overthrow the government.
Opposition parties routinely accuse the government of harassment and say their candidates were intimidated during local elections in April last year. The government denies it.