In a live television broadcast on Saturday night, President Ashraf Ghani addressed the nation following the deadly Daesh suicide bombing in Kabul city earlier in the day - which claimed the lives of at least 80 people, TOLO News reported.
Ghani condemned the attack that also claimed the life of at least one journalist.
He said: "I will get revenge on those who shed the blood of our citizens."
Ghani said it was a tragic day and he ordered Sunday be a day of national mourning.
He said: "Our enemies, by attacking civil liberties, think that they can divide us. But they will fail."
Ghani confirmed that one of three suicide bombers, that had joined demonstrators at the mass rally Saturday, had been gunned down by security forces before he could detonate his explosives.
"I have directed a special commission and the attorney general to investigate the incident and find those responsible," he said.
The Ministry of Interior on Saturday night confirmed the death toll had risen to 80 and that over 230 people were wounded.
United Nations Assisted Mission in Afghanistan (UNAMA) also condemned the attack against a peaceful demonstration.
In a statement issued by the organization, it said the two explosions occurred at Dehmazang Circle in Kabul city "targeted a peaceful demonstration, causing horrendous levels of civilian harm."
"This attack is particularly heinous because it targeted civilians as they exercised their rights to freedom of assembly and freedom of expression," the statement read.
"An attack deliberately targeting a large, concentrated group of civilians amounts to a war crime. This incident is an outrage that cannot be justified. It is an attempt to spread terror amongst civilians and stifle the freedoms that Afghans have sacrificed so much to obtain. The United Nations stands firmly with the people of Afghanistan as they seek to exercise their fundamental human rights," said UNAMA chief Tadamichi Yamamoto.
"I reiterate that international humanitarian law prohibits the deliberate targeting of civilians and international human rights law requires the government and non-state actors to respect and protect the rights to freedom of assembly and freedom of expression. The perpetrators of today's attack must be held accountable."
Resolute Support also strongly condemned the attack.
"Our condolences go out to those who are affected by today's attack," said Resolute Support Commander Gen. John W. Nicholson.
"We strongly condemn the actions of Afghanistan's enemies of peace and remain firmly committed to supporting our Afghan partners and the National Unity Government."
The Resolute Support commander also praised Afghan law enforcement officials and medical personnel whose efforts prevented additional casualties.
The United States Embassy in Kabul also spoke out against the attack.
In a statement issued by them, the embassy said it condemns "in the strongest terms the cowardly bombing attack that has claimed scores of innocent lives in Kabul during a peaceful demonstration in which citizens were exercising their constitutional rights."
Daesh claimed responsibility for the attack earlier Saturday.
The incident happened after thousands of demonstrators gathered at the Dehmazang Circle in the capital to protest against government's planned routing of the 500kV power line project.