The blast struck in front of a high school in the downtown area of the Afghan capital, the interior ministry said.
A photo shared on the popular WhatsApp messaging app showed several bodies laying on the ground.
“It was a huge blast near Istiqlal high school, very close to where the demonstrators were gathering,” witness Qais Nawabi told AFP.
A police officer at the scene said he could see 10 to 15 casualties on the ground, as well as body parts.
“It is hard to see whether it was a suicide attack or a bomb (that had been planted),” he told AFP on the condition of anonymity.
Hundreds of protesters, including university students, had taken to the streets of Kabul to demand the deployment of reinforcements to Hazara-dominated districts in Ghazni province which have been attacked by the Taliban.
AFP