The official leading the probe into corruption allegations against South Africa's police chief has himself been charged with corruption.
Police chief Jackie Selebi denies links to organised crime
Gerrie Nel, a member of an elite anti-crime unit known as the Scorpions, was freed on bail after his arrest.
His boss says the arrest will not affect the inquiry into police chief Jackie Selebi, who also heads Interpol.
A BBC correspondent says it is a further twist in rivalry between the Scorpions and the police.
Mr Selebi has denied allegations of links to organised crime.
Correspondents say questions remain about Mr Selebi's alleged close links to businessman Glen Agliotti, who is accused of involvement in the murder of mining magnate, Brett Kebble.
President Thabo Mbeki, who is said to be close to Mr Selebi, has consistently resisted calls for his police commissioner to be fired, saying he would not take any action until evidence of any wrongdoing was brought before him.
Mr Nel, who heads the Scorpions in Gauteng province, was arrested at his home in Pretoria on Tuesday night.
The Scorpions have a fierce rivalry with the police
He is alleged to have interfered in an investigation against one of his officers two to three years ago.
The Scorpions consists primarily of investigators who work hand in hand with prosecutors in the National Prosecuting Authority (NPA).
Acting NPA head Mokotedi Mpshe said the arrest was completely unrelated to the Selebi inquiry, and would have no bearing on its outcome.
He added that a decision on whether to prosecute Mr Selebi would be taken by the end of the month.
A warrant was issued for the police chief in September last year but was subsequently cancelled.
Dianne Kohler Barnard, a spokesperson for the main opposition Democratic Alliance, said the arrest "has all the appearance of a witch hunt to protect Selebi".
The BBC's Mpho Lakaje in Johannesburg says that, since its creation in 1999, the FBI-style Scorpions unit has been controversial.
It operates independently of the South African Police Service to fight organised crime.
The NPA recently charged Jacob Zuma, the new leader of the ruling African National Congress and the party's presidential candidate, with corruption, fraud, money laundering and racketeering.
Our correspondent says Mr Zuma's supporters want the unit to be dissolved and brought under the control of the police.