Four Nigerian policemen, accused of setting up an illegal checkpoint to extort money from drivers, picked on the wrong man: the head of one of the country's anti-corruption agencies.

Musa Aliyu, who leads the Independent Corrupt Practices and Other Related Offences Commission, has alleged that he was stopped while driving in the capital , Abuja, on Saturday, and forced to withdraw 53,000 naira (£28; $38) from a cash machine.

Police Commissioner Ahmed Sanusi accused the men of abandoning their official posts to set up the unauthorised roadblock.

They now face a police disciplinary procedure that could result in their sacking. It is rare that these kinds of cases end up in court.

Aliyu's report of the incident to the police prompted what the commissioner described as an "intelligence-driven operation". That led to the arrest of the alleged ringleader and subsequently the other suspects.

In an interview with BBC Hausa, Aliyu explained what happened after he was flagged down on a busy road.

"When I stopped, a policeman came to my car door. I wound down the window, and he told me to open my car, so I did."

The policeman then got in.

"He then asked me for my driver's licence. I brought it out, and he asked me to give it to him, so I did. Then he said he would take me to court for violating traffic rules. I told him: 'Let's go to court.'"

But instead, Aliyu alleged, the policeman asked him for money by telling him: "You are a big man; you wouldn't want your time to be wasted, but even if we go to court, you will be fined 52,000 naira."

"I asked him to give me his bank account number so I could transfer the money, but he refused." Instead, Aliyu alleged, he went to a cash machine and withdrew the money.

The national police chief condemned the officers' alleged conduct in strong terms, warning that the command would no longer tolerate abuses of power.

"You have to choose: either a policeman or a thief," Sanusi said. "You cannot be both. If you fail to choose, your actions will determine where you belong."