His request for a cigarette was rejected out of hand as was his attempt to plead his case with a senior officer. The exchanges were sharp if predictable.

But this week the case of Fahmi Hassan plunged Scotland Yard into a race row as previously confidential surveillance tapes emerged revealing what officers were saying privately about the Somalian youth worker as he languished in his cell.

On the tape, which the Guardian has seen, one clearly agitated supervisory officer says Mr Hassan has "lots of wants, lots of needs, lots of arrogance; nothing a good beating wouldn't put right. Knows his rights, knows the law."

The 23-year-old British born community worker is derided as an "arrogant shit" and an "obnoxious Somalian".

An officer tells colleagues in the custody suite that the prisoner is "an absolute knobhead". The sergeant adds: "There is a great film. Have you seen the film Black Hawk Down about an American helicopter that gets shot down in Somalia? Mogadishu. It is based on the truth. It is when the Americans very foolishly went into Somalia to suppress the warlords. There is no one in charge in Somalia. It's just tribal factions. There's no bugger in charge in Somalia.

"He's a Somalian. They are very violent people I think. If you think about it, the ones who got out of Somalia are either the middle classes with money or the kids with guns. It's that type of environment."

Later, discussing the time when Mr Hassan should be permitted to see a police doctor, the officer says: "he's a piece of low life". He goes on to say: "Because he is a complainer I want him seen by a doctor. He's an obnoxious Somalian. I'm sure there must be nice Somalians but he's an obnoxious ... didn't want to see the doctor, now he wants to see the doctor. Fuck him."

Mr Hassan, a volunteer with a group which aims to keep youngsters away from drugs and crime, is suing the Met alleging that he was wrongfully arrested, assaulted and maliciously prosecuted following his arrest in north London in 2005.

No drugs were found on him and he was cleared at a magistrates court last year of assaulting a police officer. The youth worker claims the supervisory officer set the tone for how he was treated by others at the police station where he says he was manhandled, humiliated and suffered racial abuse. The Met was legally obliged to hand over hours of tape from the custody suite as part of the civil action process.

Mr Hassan, who first viewed the tape on Monday, said he was shocked to hear the comments. "I couldn't believe what I was hearing. I was frustrated because I was trying to say that I had been wrongfully arrested. I had done nothing wrong. You just can't make up lies about people and arrest them. Imagine being holed up in a cell when you haven't done anything wrong."

Dan Rubinstein, his solicitor, said the tape showed clear stereotyping and that his client's treatment was influenced by the allegation officers had made against him.

"What is clear is that back at the police station, the custody officer didn't bother to look at the issue dispassionately," he said. "They automatically took the arresting officer's version of events as the truth whereas their job is to query the evidence. The custody sergeant's duty is to ensure the welfare and safety of the person detained. This case gives me very little confidence in their ability to do that."

The tapes reveal conversations in the custody suite that are alternately comic and disturbing. As Mr Hassan, who has previous drug possession convictions, sits in his cell, officers discuss subjects ranging from "black on black shootings" to the quality of Babybel processed cheese.

After fingerprinting him, officers discuss Mr Hassan's request for a cigarette. "Assaulted police? And then he wants a fag?" the supervisory officer says. "A cigarette after he's been very horrible," adds a PC. "I don't think so. Put him back in his cell," says the supervisory officer. "Ooh I love you sometimes," a WPC chips in.

There are strict rules governing the conduct of officers and their behaviour towards prisoners in custody suites. The role of the custody sergeant was created as part of the Police and Criminal Evidence Act 1984, which dictates how detainees should be treated and how long they can be held. The custody sergeant is a powerful figure because they carry the legal responsibility for prisoners in their care.

A spokeswoman for the Met confirmed that legal proceedings had been initiated and said the force would defend itself against Mr Hassan's claim.