No group has so far claimed responsibility for the blast, but it comes in the middle of the Taliban's annual summer offensive, which the insurgents are ramping up after a brief lull during the recent holy fasting month of Ramadan.
The scene of the blast was littered with charred bodies and dismembered limbs, with ambulances struggling to reach the scene as authorities had overnight blocked key intersections with stacked shipping containers to impede movement of the protesters.
"At least 20 people have been killed and another 160 others wounded, but these are initial figures from our hospitals and could change," health ministry spokesman Mohammad Ismail Kawoosi told a news agency.
Thousands of demonstrators had gathered to demand that a multi-million-dollar power line pass through their electricity-starved province of Bamiyan, one of the most deprived areas of Afghanistan with a large Hazara population.
"The horrific attack on a group of peaceful protestors in Kabul demonstrates the utter disregard that armed groups have for human life," Amnesty International said in a statement.
"Such attacks are a reminder that the conflict in Afghanistan is not winding down, as some believe, but escalating, with consequences for the human rights situation in the country that should alarm us all."
In a statement, Afghan President Ashraf Ghani said he was "deeply saddened" by the carnage, adding that the casualties included security officials.
"Peaceful protest is the right of every citizen, but opportunist terrorists infiltrated the crowds and carried out the attack, killing and injuring a number of citizens including some security forces."
The protest march was largely peaceful before the explosion struck as the demonstrators sought to march on the presidential palace, waving flags and chanting slogans such as "death to discrimination".
The 500-kilovolt TUTAP power line, which would connect the Central Asian nations of Turkmenistan, Uzbekistan and Tajikistan with electricity-hungry Afghanistan and Pakistan, was originally set to pass through the central province.
But the government re-routed it through the mountainous Salang pass north of Kabul, saying the shorter route would speed up the project and save millions of dollars.    

Hazara leaders in the ethnically divided nation lashed out at the Pashtun president, calling the decision prejudiced against the Hazaras, a community that has suffered a long history of oppression.
Today's protest follows a similar demonstration in May, which drew tens of thousands of people. The three million-strong Hazara community has been persecuted for decades, with thousands killed in the late 1990s by Al-Qaeda and the mainly Pashtun Sunni Taliban.

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