Dozens of women from Afghanistan's minority Hazara community protested in the capital Saturday after a suicide bombing a day earlier killed 20 people — mostly young women from the ethnic group.

A bomber blew himself up on Friday at a Kabul study hall as hundreds of pupils were taking tests in preparation for university entrance exams in the city's Dasht-e-Barchi area.

The western neighborhood is a predominantly Shiite Muslim enclave and home to the minority Hazara community — a historically oppressed group that has been targeted in some of Afghanistan's most brutal attacks in recent years.

Police said at least 20 people were killed but the United Nations has put the number at 24.

On Saturday about 50 women chanted, "Stop Hazara genocide, it's not a crime to be a Shiite", as they marched past a hospital in Dasht-e-Barchi where several victims of the attack were being treated.

Dressed in black hijabs and headscarves, angry protesters carried banners that read: "Stop killing Hazaras", an AFP correspondent reported.

They later gathered in front of the hospital and chanted slogans as dozens of heavily armed Taliban fighters kept watch.

Since the hardline Taliban returned to power last August, women's protests have become risky, with numerous demonstrators detained and rallies broken up by Taliban forces firing shots in the air.

No group has claimed responsibility for Friday's attack at the Kaaj Higher Educational Centre. But the jihadist Islamic State (IS) group regards Shiites as heretics and has previously claimed attacks in the area targeting girls, schools and mosques.

The Taliban also regard the Hazara community as heathens, and rights group often accused the Islamists of targeting the ethnic group during their 20-year insurgency against the former US-backed government.

Since returning to office, the Taliban have pledged to protect minorities and clamp down on security threats.

However, rights group Amnesty International said the Friday attack was "a shamefaced reminder of the inaptitude and utter failure of the Taliban, as de-facto authorities, to protect the people of Afghanistan".

In May last year, before the Taliban's return to power, at least 85 people — mainly girls — were killed and about 300 were wounded when three bombs exploded near their school in Dasht-e-Barchi.

No group again claimed responsibility, but a year earlier IS claimed a suicide attack on an educational center in the same area that killed 24.

IS has emerged as a key security challenge for the Taliban, but officials claim their forces have defeated the jihadists.