South African police have arrested four right-wing extremists suspected of plotting an attack on a conference of the ruling party.

Police issued a statement Monday, but gave few details about the arrests, saying they occurred Sunday and that the men are between 40 and 50 years old.

The African National Congress, which has led South Africa since the end of white minority rule in 1994, is holding a five-day conference in the city of Bloemfontein.

ANC spokesman Keith Khoza said Monday the party is still waiting for a full report on the arrests, calling the plot a “disturbing matter.” He said race-based threats are worrying, but that the meeting will continue under heavy security.

“We are concerned because you have elements in society that still think in that way. It says you are facing serious challenges. South Africa has been through the worst and the last thing we want is to go back to a situation where you still experience race-based violence or terrorism. It is not in keeping with what we as South Africans collectively are trying to achieve.”

A fringe party fighting for self-determination for the white Afrikaner minority, the Federal Freedom Party, said in a statement that two of its members are among those arrested. A party official said the group is not associated with any plot.