ISTANBUL — In a narrow side street of Istanbul the riot police, with gas masks pushed up on their heads, advanced on a small band of demonstrators marching in silence.

The police were there to stop a collection of aging mothers and their relatives, who were protesting the disappearances and extrajudicial killings decades ago of their family members, mostly activists from Turkey’s Kurdish minority.

Holding photographs of their missing relatives, the protesters, known as the Saturday Mothers, sat down briefly on the ground before dispersing as a police commander threatened over a bullhorn to break up the gathering by force.

The aggressive response to a peaceful, long-established protest by grieving mothers was seen by government critics as evidence of a continuing turn toward authoritarianism in Turkey under President Recep Tayyip Erdogan — a leader who had once pledged to help these families find their loved ones.