South African police have foiled a plot by suspected right-wing Afrikaner extremists to bomb a marquee at an African National Congress (ANC) conference attended by President Jacob Zuma and dozens of top government officials, according to Reuters.

A police statement released on Monday said four men aged between 40 and 50 were arrested on Sunday, Dec 16.

The statement gave few other details, but national police spokesman Phuti Setati said they had been planning to plant a bomb in one of the tents at the ANC conference being held in the central city of Bloemfontein.

The Federal Freedom Party (FFP), a fringe group fighting for self-determination for South Africa's white Afrikaner minority, confirmed two of those arrested were FFP members but denied any role in the suspected plot.

"We were not involved and do not associate ourselves with their actions," FFP national secretary Francois Cloete said.

More than 4,500 people, including Zuma and dozens of ministers and top business people, are attending the five-day conference in Bloemfontein under a heavy police presence.