The release of Merera had been a key demand of protesters from Ethiopia's largest ethnic group, the Oromos.

In 2015, the Oromos took to the streets over a proposed expansion of the capital city, Addis Ababa. They feared that this would deprive them of their land.

Hundreds were killed in the unrest, leading to a 10-month state of emergency that was lifted in August.

Merera is the first political prisoner to be released since Ethiopia's Prime Minister Hailemariam Desalegn announced earlier this month that convicted politicians would be pardoned. 

On Monday, Attorney General Getachew Ambaye told journalists that 528 people had so far been selected for clemency, including Gudina, leader of the opposition group Oromo Federalist Congress (OFC) who was arrested in late 2015.

Getachew said the criteria for their selection involved taking into account evidence that the suspects did not take part in actions that led to killings and severe injury, damaging infrastructure, and "conspiracy to dismantle the constitutional order by force."

Prime Minister Desalegn announced earlier this month that dissident politicians jailed on criminal charges would be released and pardoned in a bid to foster reconciliation.

"Politicians currently under prosecution and those previously sentenced will either have their cases annulled or be pardoned," he said.

"And the notorious prison cell that was traditionally called Maekelawi will be closed down and turned into a museum," he added.

The announcement came after recent anti-government protests in the troubled Oromia and Amhara regions brought business and transport networks to a standstill.

The deadly protests began in late 2015 and led to tens of thousands of arrests and the displacement of some 1 million people.

Ethiopia is often accused by rights groups of using security concerns as an excuse to stifle dissent and media freedoms.