South Africa's national police chief says her forces opened fire in self-defense against striking mine workers, in violence that left 34 people dead and 78 injured.

Riah Phiyega said Friday that police used force to protect themselves after coming under attack Thursday by strikers armed with “dangerous weapons.” The mining company also said after the incident that some of the strikers were armed.

Phiyega 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, about 120 kilometers northwest of Johannesburg.

South African President Jacob Zuma said he was “shocked and dismayed” at what he called “senseless violence.” He stopped short of saying who was responsible and urged unions to work with the government to address the situation.

Mr. Zuma cut short a visit to a regional summit in Mozambique Friday to address the incident.

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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