According to the website of opposition Ethiopian Satellite Television (Esat), Mr Merara was in Brussels where he testified at the European parliament on the current political crisis and human rights violations in Ethiopia.
He had called for Ethiopia’s parliament to be dissolved and for the establishment of a transitional government, Esat reports.
Media outlets affiliated with the government had been campaigning for his arrest, saying he had violated the current state of emergency by attending the hearing at the European parliament, it added.
Ethiopia declared a six-month state of emergency last month in the face of an unprecedented wave of protests by members of the Oromo and Amhara ethnic groups.
Read more: Seven things banned under the state of emergency