A plot to bomb the national conference of South Africa's governing African National Congress has been foiled, a police spokesman has said.
Four white suspected right-wing extremists have been arrested, he said.
President Jacob Zuma and other top officials are at the heavily-guarded conference in Mangaung, where the ANC is due to start electing its leaders.
The ANC has been in power in South Africa since the end of white minority rule in 1994.
Its leader will be overwhelming favourite to win elections due in 2014.
Police spokesman Phuti Setati told Reuters news agency the four suspects were arrested for planning to plant a bomb in a marquee at the conference, which is being held at the University of the Free State in Mangaung.
South Africa's City Press newspaper reports that the arrests took during police raids in three of South Africa's nine provinces - Free State, Northern Cape and Limpopo.
The Federal Freedom Party (FFP), which campaigns for the self-determination of South Africa's white Afrikaner minority, confirmed that two of those arrested were its members.
However, it denied any role in the alleged plot.
"We were not involved and do not associate ourselves with their actions," FFP national secretary Francois Cloete told Reuters.
The BBC's Milton Nkosi reports from the conference that there is a strong security presence, with police and sniffer dogs checking all cars entering the campus.
In July, a South African court convicted former academic Mike du Toit of treason.
The court ruled that Du Toit, the leader of the Boeremag (Afrikaner power) group, was behind a spate of bombings in 2002 and had also plotted to assassinate South Africa's first black president, Nelson Mandela.
Mr Zuma, who opened the conference on Sunday, is standing for re-election as ANC leader, despite a promise before he became party leader in 2007 that he would step down after one term.
He is being challenged by his deputy, Kgalema Motlanthe.
Mr Zuma is the favourite to win.
One of South Africa's richest businessmen, Cyril Ramaphosa, has lent his support to Mr Zuma by standing for the party's deputy leadership, our correspondent says.
Mr Ramaphosa played a key role in the transition to black-majority rule.
In his opening address in Mangaung, where the ANC was formed a century ago, Mr Zuma said the country was now ready to "move into the second phase, in which we will focus on achieving meaningful socio-economic freedom".
The results of the leadership contest are expected later this week.