At least 61 people have been killed and 207 wounded when two suicide bombers detonated explosives at a large demonstration in Kabul on Saturday afternoon, Health Ministry spokesman Ismail Kawusi said.
Islamic State has claimed responsibility for the attack, writing on the group-affiliated Aamaq News Agency that two of their fighters carried out the attack on protesters in the Dehmazang area of Kabul.
Afghan security officials have yet to comment on the incident.
At least 10,000 people attended the protest, according to event organizer Mohammad Arif Rahmani.
Protesters were rallying against the construction of an electrical transmission line from Turkmenistan to Pakistan via Kabul, demanding it be rerouted through the central Afghan province of Bamyan, which is populated with minority Hazaras.
The Afghan government had rejected the demands, citing further delays and added costs.
Afghan President Ashraf Ghani said he is "deeply saddened" by the attack, adding that Afghan security forces are among the casualties.
Afghan Chief Executive Abdullah Abdullah condemned the "terrorist attack."
Pictures on social media show horrific scenes with scores of people wounded in the square where the protesters had gathered.
Taliban militants denied involvement in the attack.
"We want to make it clear," Taliban spokesman Zabihullah Mujahid wrote in an email. "Our Mujahedeen had no hand in the attack."