A protest march was held in Brighton yesterday following the death of Mohamud Mohammed Hassan last month.
The 24-year-old was arrested at his home in Cardiff on suspicion of breach of the peace on January 8 but was released from police custody without charge the following morning.
He died later that day.
His family say that, before he died, Mr Hassan claimed he was assaulted in custody.
South Wales Police said it found no evidence of excessive force but had referred itself to the Independent Office for Police Conduct (IOPC).
A South Wales police officer was served with a misconduct notice last week as part of an investigation by the IOPC into Mr Hassan's death.
In Brighton yesterday afternoon, a group of about 50 people gathered by the Clock Tower in Brighton before making their way to Brighton Police Station in John Street.
Passers-by say they heard protesters chanting "the UK is not innocent" and "no justice, no peace, no racist police".
Outside the police station, they sat in the road and held a minute's silence before being moved on by police.
A South Wales police officer was served with a misconduct notice on February 15 as part of an investigation by the IOPC into the death of Mohamud Mohammed Hassan.
The IOPC said the officer was in the back of a police van with Mr Hassan when the 24-year-old was "heard on body-worn camera to complain of having a fit, suffering a migraine, and displayed signs of experiencing pain".
IOPC director for Wales Catrin Evans said: "We are continuing to analyse the footage and piece together other evidence, and we are looking at all the interaction police had with Mr Hassan over the weekend of his death.
"In the course of an investigation, where an indication arises that an officer may have breached professional standards that may warrant a disciplinary sanction, we serve a disciplinary notice to advise them they are subject to investigation.
"We have advised Mr Hassan’s family and South Wales Police that we have done so for one officer over possibly not passing information about Mr Hassan’s welfare to the custody sergeant on duty.
"We keep misconduct notices under review during the course of an investigation.
"At the conclusion of an investigation the IOPC decides whether any officer under notice has a disciplinary case to answer."
An independent investigation into the circumstances surrounding Mr Hassan's death is ongoing, with body-worn video and CCTV video being analysed by the IOPC.
Following the death of Mr Hassan, hundreds took to the streets of Cardiff for several days of protests, with many asking for answers from police on how and why he died.
At First Minister's Questions on January 12, ITV News reported that Plaid Cymru leader Adam Price called on Mark Drakeford, the First Minister of Wales, to provide Mr Hassan's family with answers to questions surrounding his death.
He said: "Mohamud Hassan was a fit and healthy 24-year-old.
"On Friday evening (January 8), he was arrested at a property in Cardiff where neighbours reportedly spoke of a significant commotion.
"Having been taken into custody at Cardiff Bay police station, Mr Hassan was released without charge on Saturday. Later that evening he tragically died.
"Witnesses were reportedly shocked by Mr Hassan's condition following his release, saying that his tracksuit was covered in blood and he had severe injuries and bruising.
"There can be no doubt that this is a deeply harrowing case and every effort should be made to seek the truth of what happened - why was Mohamud Hassan arrested, what happened during his arrest, did he have legal representation, was there any aftercare, why did this young man die?
"While we should not prejudge the outcome of any inquiry, will you commit First Minister, to doing everything within your power to help the family find those answers and do you support their call for an independent investigation of this case?"
Mr Drakeford said he found the case "deeply concerning".
He said: "If there are things the Welsh Government can do, then I will make sure that we attend properly to those, without prejudging in any way the outcome of the independent investigations and now need to follow."
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