Thirty-four people were killed after police opened fire on striking miners at a South African mine on Thursday, the police chief has said.
Riah Phiyega said police had been forced to shoot after armed protesters charged them, "firing shots", at the Marikana mine in the north-east.
At least 78 people were injured in the confrontation, she added.
Unions are demanding an inquiry into the incident - one of the bloodiest police operations since apartheid.
The Lonmin-owned platinum mine has been at the centre of a violent pay dispute, exacerbated by tensions between two rival trade unions.
Violence had already killed 10 people, including two police officers, since the strike began a week ago.
Some of the strikers' wives gathered near the mine on Friday, chanting anti-police songs and demanding to know what had happened to their husbands.
"Police, stop shooting our husbands and sons," read a banner carried by the women, according to the Associated Press news agency.
South African President Jacob Zuma has cut short a trip to Mozambique in order to visit the mine, which lies about 100km (62 miles) north-west of Johannesburg.
Police were sent to break up some 3,000 miners - some armed with clubs and machetes - who had gathered on a hillside overlooking Marikana to call for a pay rise of about $1,000 (£636) a month.
The circumstances that led police to open fire remain unclear, but reports from eyewitnesses suggest the shooting took place after a group of demonstrators rushed at a line of police officers.
Police, armed with automatic rifles and pistols, fired dozens of shots, witnesses said.
One witness, Molaole Montsho, of the South African news agency Sapa, said police had first used water cannon, tear gas and stun grenades in an attempt to break up the protest.
"And then in the commotion - we were about 800m [2,600ft] from the scene - we heard gunshots that lasted for about two minutes," he said.
David Nkolisi, 37, who works as a rock-drill operator, told the BBC: "We were killed for asking our employer to pay us a decent salary for hard work deep underground."
At a news conference on Friday, Ms Phiyega said police had been forced to open fire to protect themselves.
"The militant group stormed toward the police, firing shots and wielding dangerous weapons," Ms Phiyega said. "Police were forced to use maximum force to defend themselves."
She said 259 people had been arrested on various charges.
The Association of Mineworkers and Construction Union (AMCU) accused the police of carrying out a massacre.
"There was no need whatsoever for these people to be killed like that," General Secretary Jeffrey Mphahlele told Reuters news agency.
A spokesman said President Zuma would be travelling to the site later on Friday.
"The president is concerned about the violent nature of the protest, especially given that the constitution and labour laws allow enough avenues to deal with issues, and is sympathetic to calls for a commission of inquiry," his spokesman Mac Maharaj said in a statement.
Thursday's incident came after several days of violent strikes in which 10 people were killed, including two police officers who were hacked to death.
The miners, who are currently earning between 4000-5000 rand ($484 - $605), say they want their salary increased to 12,500 ($1,512).
The stand-off has been exacerbated by rivalry between two trade unions, with the AMCU, a new group, seeking to challenge the dominance of the NUM.
The NUM is seen as being close to the ruling African National Congress (ANC) and the two groups were allies in the fight to end white minority rule, although relations between the ANC and the unions have worsened in recent years.
South Africa is the largest platinum producer in the world and the dispute has already affected production.
Lonmin, the world's third-largest platinum producer, has encountered similar labour disputes at the Marikana mine.
In May 2011, the company sacked some 9,000 employees after what it described at the time as "unprotected industrial action". Lonmin and the NUM said all were later reinstated.