The state-run news agency report on Tuesday came just one day after 43 opposition members had been detained for alleged subversion in northern Tigray state.
The official Ethiopian News Agency (ENA) said dozens more had been arrested on similar charges in southern Oromia state and another three south of the capital.
In a brief item that offered few specifics, the agency quoted Oromia police as saying they had arrested 80 people believed to be opposition supporters had seized 160 illegal weapons in raids in the towns of North and East Shoa.
"The illegal arms and the people were apprehended through house-to-house searches after the police were tipped by members of the public," ENA quoted North Shoa deputy police commander Tesfaselassie Negera as saying.
The report said the confiscated weapons included hand grenades, hand guns, assault rifles and ammunition while various communications equipment was also seized from the suspects.
It added that at least eight of the detainees were affiliated with the Coalition for Unity and Democracy (CUD), one of the country's two main opposition groups planning a weekend rally to protest disputed May elections.
The agency also reported that three people had been arrested and 28 illegal weapons and various ammunition seized from a private car just south of Addis Ababa.
The arrests, for which ENA did not provide dates or other details, were announced as opposition groups prepare for Sunday's demonstration with vigorous denials of the government's coup plot allegations.
The opposition groups have called the rally to protest official results from the 15 May polls which give the ruling Ethiopian Revolutionary Democratic Forces (EPRDF) a victory and to press home their call for the formation of a national unity government to oversee new elections.
The authorities, which have rejected the opposition demands, have accused the groups of planning to foment violence to overthrow the government and warned of severe consequences for any illegal acts that result from the demonstration.
Protests over the election results have already turned violent, with at least 36 and as many as 42 people killed when police opened fire on crowds during demonstrations in the capital in June.