South Africa's governing ANC is not attending commemorations marking the death of 34 striking miners shot by police a year ago, saying they have been "hijacked" for political reasons.
Several opposition leaders, including firebrand youth leader Julius Malema, are sitting with the miners' widows.
The government was criticised for its handling of the crisis.
It was the most deadly use of police force since the ANC came to power in 1994, ending white minority rule.
The killings shocked South Africa and hit confidence in its mining sector.
The country is home to about half of the world's known gold reserves and 80% of its platinum.
Rivalry between two unions - one allied to the ANC - was at the heart of last year's unrest.
The pro-ANC National Union of Mineworkers has been displaced as the majority union in the Marikana mine.
"The ANC finds it immoral and totally unacceptable that such a traumatic incident could be turning into a political playground" by its opponents, it said in a statement.
There are also no government officials at the events, organised by the militant Association of Mineworkers and Construction Union (Amcu), which called last year's unofficial strikes and is again pushing for substantial wage increases.
It is now the biggest union at the Lonmin-owned Marikana platinum mine.
Some in the ANC fear that Amcu could form an alliance with Mr Malema, who was expelled from the party last year and who recently formed a new political grouping.
Mr Malema, who fell out with South African President Jacob Zuma and accused him of ignoring poor black South Africans, was loudly cheered when he arrived.
About 1,000 mine workers have gathered at the rocky hill, or koppie, where their colleagues were killed last year, singing songs and listening to speeches.
Religious leaders are holding prayers and there will be a minute's silence shortly after 14:00 GMT - around the time when the police opened fire on a group of striking miners.
The police say they feared they would be attacked by a mob armed with machetes, sticks and other weapons.
In the preceding week, 10 people had been killed during violent protests, including two security guards and two police officers.
President Zuma set up a commission of inquiry to investigate the events leading to the killings.
But the BBC's Andrew Harding in South Africa says there is anger in Marikana that not a single policeman has yet been arrested over the shootings.
He says that while talks between the government, unions and business have yet to produce stability, an atmosphere of violence and intimidation endures in the area.
Earlier this week, a senior NUM official was shot dead near the mine.