National Prosecuting Authority (NPA) spokesman Tlali Tlali told Reuters the NPA would oppose court papers designed to block "possible prosecution" of Selebi.

Selebi, who has denied allegations of links to organised crime and corruption, has filed an urgent application to the Pretoria High Court in a personal capacity, his spokeswoman Sally de Beer said. She said Selebi did not want to comment.

Selebi, who is also the president of Interpol, has come under fire over his friendship with convicted drug smuggler Glenn Agliotti, who is accused of playing a role in the 2005 murder of mining magnate Brett Kebble.

President Thabo Mbeki has stood by Selebi despite calls by the opposition for his dismissal, prompting allegations he was being protected for political reasons.

POLITICAL BATTLE

Corruption allegations are explosive in South Africa, where Mbeki and his rival Jacob Zuma -- new leader of the ruling African National Congress -- are locked in a power struggle that has raised investors' fears of political instability.

The NPA has also charged Zuma with corruption, fraud, money laundering and racketeering. He is due to stand trial in August.

Senior ANC figures have criticised the NPA, saying there was a political motivation for the charges against Zuma, who beat Mbeki in the party leadership race last month. Their rivalry has plunged the ANC into the worst infighting in its history.

The NPA charged Zuma less than two weeks after he took over from Mbeki, fuelling allegations it has been used as a political weapon.

Gerrie Nel, who is leading the investigation into Selebi, was arrested on graft charges by policemen on Tuesday night.

Nel, a senior official of the Scorpions elite crime-fighting unit, has been released on bail and will appear in court on Monday, his lawyer, Ian Small-Smith, told Reuters. The charges against Nel include fraud and "defeating the ends of justice".

Selebi's bid to block his possible prosecution aroused fresh suspicions from the opposition Democratic Alliance.

"For someone who has always claimed that he is innocent and has been quoted as stating that he will never be arrested, he was pretty quick off the mark with his application," Dianne Kohler Barnard, a party spokeswoman, said in a statement.