Prime Minister Hailemariam Desalegn yesterday said Ethiopia had a moral obligation to arrest the Andargachew Tsige, the secretary-general of Ginbot 7, who was extradited from Yemen.
In an exclusive interview with the British Broadcasting Corporation's (BBC) Focus on Africa, PM Hailemariam said: "His stooges were there, who were sent to destabilize and bomb this country - they were captured red-handed with their bombs and their detonating elements."
Asked if the death penalty would be carried out now that Andargachew was in custody, the prime minister replied: "I cannot say this now."
Hailemariam said Ethiopia was a young democracy and opposition parties were allowed to operate, but terrorism would not be tolerated.
"If you ask me to tolerate in the name of democracy those who are engaging [in] destabilizing the country and acting as a terrorist and using guns to change government, then you are wrong," he said.
"If you have any connection with terrorists don't think that the Ethiopian government will let you [go] free," he added.
Andargachew Tsege, a UK national, was sentenced to death in 2009 while in exile for plotting a coup.
The group says Andargachew was on his way from the United Arab Emirates to Eritrea when he was detained at Sanaa airport on 24 June.
Foreign governments could express their concern, but the man would be dealt with according to the law, the PM said.