JERUSALEM — Ethiopian-Israelis and their supporters took to the streets across the country on Wednesday for a third day of protests in an outpouring of rage after an off-duty police officer fatally shot a black youth, and the Israeli police turned out in force to try to keep the main roads open.

The mostly young demonstrators have blocked major roads and junctions, paralyzing traffic during the evening rush hour, with disturbances extending into the night, protesting what community activists describe as deeply ingrained racism and discrimination in Israeli society.

Scores have been injured — among them many police officers, according to the emergency services — and dozens of protesters have been detained, most of them briefly. Israeli leaders called for calm; fewer protesters turned out on Wednesday.

“We must stop, I repeat, stop and think together how we go on from here,” President Reuven Rivlin said on Wednesday. “None of us have blood that is thicker than anyone else’s, and the lives of our brothers and sisters will never be forfeit.”