MARIKANA, South Africa — South Africa’s police commissioner on Friday defended the actions of officers who opened fire on miners a day earlier during a wildcat strike at a platinum mine. She said the episode left 34 people dead and 78 wounded, a sharply higher toll than initially reported.

The commissioner, Riah Phiyega, described a desperate struggle by the police to contain the machete-wielding crowd of thousands of angry miners who broke through two lines of defense, leaving officers with no choice but to open fire with live ammunition.

“The militant group stormed toward the police firing shots and wielding dangerous weapons,” Ms. Phiyega said at an emotional news conference here, using an extensive array of aerial photographs and video to demonstrate how the violence unfolded. Previous attempts by the 500-strong police force to repel the crowd with rubber bullets, water cannons and stun grenades had failed, she said.