Abiy's sweeping reforms since his election last year have been threatened by two violent attacks on Saturday in which the army chief and a retired general were killed in Addis Ababa and three regional officials were killed in Amhara province, north of the capital.
Brig. Birhanu Jelan, the deputy army chief, speaking at the service Tuesday in Millennium Hall in the capital, said Ethiopia's security forces will fight to maintain the country's unity and stability.
In the wake of the killings Abiy has ordered a security crackdown in which the general accused of leading the plot was killed Monday and more than 180 have been arrested.
The internet has been shut down since Sunday.
Eskinder Nega, a prominent rights activist, told The Associated Press that several members of his Baladera Council group have been detained in a wave of arrests by the security forces since Saturday's killings.
Widespread arrests are taking place in Bahir Dar, the regional capital of Amhara, according to residents, who spoke on condition of anonymity for their security.
Abiy's visible distress at the service is understandable, according to experts on Ethiopia's politics.
"The assassinations were a severe setback to Abiy and a wake-up call showing the fragility of his reform plans," said Alex Vines, head of the Africa Program at Chatham House in London.
"Abiy has shown tremendous bravery in his reform agenda and in trying to dismantle the military and security regime that had run Ethiopia. These attacks show that some of those threatened by his reforms are striking back. The danger is that Abiy will respond to this by becoming more authoritarian himself."
The weekend assassinations in Addis Ababa and Amhara were "a blow to Abiy's democratization agenda and may well worsen Ethiopia's political and security crisis if key issues are not urgently tackled," said William Davison, senior analyst on Ethiopia for the International Crisis Group.
"Abiy's challenges include divisions within the ruling EPRDF party that are exacerbated by opposition challengers, including strong ethno-nationalist movements," said Davison. "The consequent dysfunction of an until recently all-powerful ruling coalition is reducing government effectiveness and contributing to the insecurity. Unless this situation improves, then it will be difficult to create a conducive environment for the planned competitive elections next year."
Meldrum reported from Johannesburg. AP journalist Rodney Muhumuza in Kampala, Uganda, contributed.
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