Thousands of government supporters gathered at Meskel Square in Addis Ababa on Thursday to celebrate the second filling of the Grand Ethiopian Renaissance Dam (GERD).
Those joining the rally also made a show support for the Ethiopian military while denouncing rebels in the country's Tigray region.
The dam has caused controversy with Egypt and Sudan being in dispute with the building and filling of it.
The issue is due to be discussed by the UN Security Council amid fears that conflict could be sparked by the project.
The demonstration, organized by the city administration of Addis Ababa, is the first public rally after the national election held in June.
Mayor Adanech Abebe spoke at the rally congratulated Ethopians' resolve at getting the dam refilled.
Abebe went on to show her allegiance with the military in the conflict in Tigray, saying she was "standing with and supporting our defense and security forces more than ever before."
The demonstrators who gathered from different parts of Addis Ababa at the Meskel Square have been very vocal in denouncing the Tigrayan People's Liberation Front, which is designated by the House of People's Representatives (HoPR) of Ethiopia as a "terrorist organization."
After eight months of fierce fighting a ceasefire came into effect some weeks ago between forces affiliated to the Ethiopian government and the TPLF.
The government has since severed transport and communication to the region that faces the world's worst famine crisis in a decade.