A suicide bomber blew himself up in the Afghan capital of Kabul on Monday, senior security officials said, but there was no immediate word on casualties.
The attacker on foot detonated his bomb close to a police checkpoint near a school in central Kabul, an official said.
A senior interior ministry official said many police officials could have been injured and ambulances were rushed to the blast site.
Kabul police spokesman Basir Mujahid said he was about 20 m (66 ft) away from the blast near where a demonstration had broken up some 30 minutes before.
"I took four bodies away but there were more on the ground," he said, without giving further details.
No militant group has claimed responsibility for the attack that came as hundreds of demonstrators gathered in Kabul to protest against the government's failure to prevent attacks by Taliban militants in two provinces.
Afghan security forces suffered scores of casualties in heavy fighting at the weekend with Taliban militants in the provinces of Ghazni and Herat, officials have said.