Merera Gudina, the 60-year-old chairman of the Oromo Federalist Congress (OFC) was arrested at his home in the Ethiopian capital Addis Ababa on Wednesday and is being held in an unknown location with three others, said Beyenne Petros, president of the Medrek opposition alliance of which the OFC is a member.
"This is the first time they are targeting the highest level of leadership. I don't fully understand. Merera has always done things peacefully and played by the rules," Petros said of the veteran political leader.
Earlier this month Gudina had addressed the European Parliament in Brussels, alongside Olympic silver medallist runner and fellow member of the Oromo tribe, Feyisa Lilesa.
Lilesa drew attention to an Oromo anti-government movement by crossing his wrists above his head -- a gesture that has become a symbol of the protests -- at the Rio games and has been in self-imposed exile since then.
Berhanu Nega, an opposition activist sentenced to death in absentia, had attended the same meeting in Brussels.
At home in Ethiopia, Gudina has strongly criticised repression of the unprecedented protests that have posed the biggest challenge to the quarter-century rule of the Ethiopian People's Revolutionary Democratic Front (EPRDF).
Hundreds have been killed in a government crackdown since the unrest began about a year ago, according to human rights groups.
A state of emergency was announced in October, a week after more than 50 people died in a stampede in the Oromia region when security forces teargassed a religious festival where protesters were chanting anti-government slogans.
Since then official figures show over 11,000 people have been arrested in the Oromia, Amhara and Addis Ababa regions where protests had been centred.
Among those arrested are leaders of small opposition parties, journalists and at least two bloggers.
Ethiopian authorities said last month that 2,000 of those detained had been released after undergoing a "re-education" and "counseling" programme.
A key complaint of the protesters is a political system which has seen the ruling party hold all 546 seats in parliament.
Ethiopian Prime Minister Hailemariam Desalegn in October promised electoral reforms, but Gudina said this was "too little, too late."