South Africa's powerful National Union of Mineworkers said it will boycott Friday's ceremony marking one year since the killing of 34 miners, amid a feud with a rival labour group.

NUM -- which is locked in a bloody turf battle with the rival Association of Mineworkers and Construction Union -- said it will shun the event because it has been "hijacked" by opposition groups.

"NUM took a firm decision to not be party to tomorrow's commemoration of the Marikana tragedy," the union said in a statement.

On August 16 last year South African police opened fire on a crowd of striking Lonmin miners, killing 34.

NUM, linked to the ruling ANC, said a commemoration planned for Friday featured "only opposition political parties" linked to AMCU.

The decision shreds hopes that NUM and AMCU could use the event to move beyond deadly violence between its members.

AMCU had earlier this week put aside its differences with NUM asking it jointly make a peace call at the commemorations.

Thousands of mine workers and sympathisers are expected to gather Friday near the deaths occurred.

Since August last year at least eight prominent members from both AMCU and NUM have been murdered.

The latest victim, a local NUM leader, was gunned down on Monday.

In the last 12 months AMCU's hardline stance on wage negotiations has enticed legions of NUM members, who accuse it of being in league with mine owners, to jump ship.

AMCU now represents 70 percent of union members among Lonmin's 27,000 workers.

NUM, once the dominant union at Lonmin, now has 20 percent.

NUM said it would "desist from organising separate commemorations as these may fuel tensions."