South African President Jacob Zuma has ordered an official inquiry into the police killing of 34 striking miners, the deadliest security operation in the country since the end of apartheid.

Mr. Zuma said he was “shocked and dismayed” at what he called “senseless violence.”

The president cut short a visit to a regional summit in Mozambique Friday and traveled to the mine in Marikana, northwest of Johannesburg. Mr. Zuma stopped short of saying who was responsible for the killings and urged unions to work with the government to address the situation.

Earlier Friday, South Africa's national police chief, Riah Phiyega, said her forces opened fire in self-defense against striking mine workers. In addition to the 34 deaths, the violence Thursday also left 78 people injured.

Phiyega said police used force to protect themselves after coming under attack by strikers armed with “dangerous weapons.”

She said police had earlier used water cannon and stun grenades to get the workers to disperse, but that they did not heed the calls.

The police chief also said 259 people have been arrested on charges related to the clash at the Lonmin PLC platinum mine.

Before the clash Thursday, Lonmin management warned workers they would be dismissed if they did not return to work by Friday.

Prior to the clash, 10 people, including two police officers, were killed in violence related to the mine strike.

About 3,000 people have walked off the job at the mine in the past week due to a pay dispute. Lonmin is the world's third-largest platinum producer.

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