JERUSALEM — An Israeli soldier who shot an incapacitated Palestinian attacker in the head was sentenced to 18 months in prison Tuesday, in a case that has divided Israelis and prompted calls for his pardon.

Colonel Maya Heller, a judge speaking for a military panel in Tel Aviv, reminded the defendant, Sergeant Elor Azaria, that his manslaughter conviction was “grave’’ and that he could have faced up to 20 years in prison.

“The acts of the defendant are serious,’’ she said, according to Israel Radio. “There is a need to safeguard humanity.’’

Azaria, now 20, was convicted in January for the death of Abdel al-Fatah al-Sharif in Hebron in March.

Video showed him shooting al-Sharif, who was lying still on his back, in the head. Al-Sharif and another Palestinian had stabbed and wounded an Israeli soldier at a checkpoint. Israeli soldiers killed the other man and wounded al-Sharif.

The case unfolded against a backdrop of random attacks by Palestinians that had killed at least 28 Israelis in a little over four months.

Conservatives expressed outrage, with Naftali Bennett, education minister, accusing the generals of being “quick to pounce on the soldier.’’ Palestinians and Israeli rights advocates said the events showed a callousness to Palestinian life.