More than 200 of the suspects are accused of killings, Amhara state security official Desalegne Tasew told the Amhara Media Corporation, accusing them of conducting "illegal activities" in the name of the Fano paramilitary group. It was not clear why the rest were being detained.
Tasew did not provide any evidence to back up the accusations of killings. Reuters could not independently verify the claims.
Gizachew Muluneh, spokesperson for the Amhara regional administration and federal government spokesperson Legesse Tulu did not immediately respond to requests for comment on the wave of arrests.
Amhara is the second-most populous region in Ethiopia, and a key constituency for Prime Minister Abiy Ahmed.
Amhara forces and the Fano militia backed Abiy's federal troops against rebellious forces in northern Tigray when fighting erupted there in 2020, but relations between some top Amhara officials and central government have since soured.