The police received information from several sources that striking miners in North West would not disperse peacefully, national police chief Riah Phiyega said on Friday.
"By midday, yesterday [Thursday] we had received information from various sources that the protesters would not end the strike peacefully and they would not leave their gathering point or disarm.
"The options were weighed and the decision taken that the SAPS [SA Police Service] needed to protect their members adjacent to the protesters," Phiyega told reporters in Rustenburg.
Asked who gave the police the order to shoot, she replied: "As commissioner, I gave police the responsibility to execute the task they needed to do."
Earlier, she said 34 people were killed in a shoot-out between the striking workers and the police on a hilltop near Lonmin's mine at Marikana in Rustenburg.
"The total death of the protesters currently stands at 34 with more than 78 injured," Phiyega said.
A total of 259 people were arrested and six firearms recovered.
"This is no time for blaming, this is no time for finger-pointing. It is a time for us to mourn...
"The police had to use force to protect themselves from the... group," said Phiyega.
The leaders of the Marikana protesters tried to no avail to get a peaceful solution throughout the week, she said.
Weapons taken off the dead protesters included firearms stolen from the two police officers who were murdered earlier in the week.
Earlier at the briefing, the police showed video footage of how two policemen were killed by a group of protesters.
The video showed police officials confronting the group, asking for weapons.
The men told the police the weapons would be handed over at the mountain, and the police followed them there.
At the hilltop, the incident turned violent and the two police officers were killed.
Journalists were also showed aerial photographs of the naked men doing a ritual with a sangoma.
The group was described as "pretty militant".