The family of an inmate who was stabbed to death at HMP Wormwood Scrubs have said conditions in the prison were "absolutely shocking".
Khader Saleh, 25, was found with a number of knife injuries on Wednesday afternoon and was pronounced dead at the scene.
Four inmates at the west London jail have been arrested on suspicion of murder.
The victim's brother Said Yousef said he had told his family about problems with gangs and violence inside the prison.
He told BBC Radio London: "I was just in a shock position since the time that I heard.
"I wouldn't believe it because someone was inside the jail and got murdered in this way, it was really shocking for me."
He added: "He said there's a lot of gangs inside and that guards are not doing anything about it.
"It's basically they let it go if fight happen."
He has called for the killers to be brought to justice: "I'm talking on behalf of our family and saying they have to get the highest sentence in the prison for the horrible things they have done."
Mr Saleh's family has asked for his body to be released by the coroner so that he can be buried in accordance with Islamic tradition.
Community worker Duale Yousef, who is supporting the relatives, told BBC Radio London: "The police told us the coroner will held another 28 days - Islamic way it has to be allowed for Muslims to bury their deceased ones."
He said the community is in shock and added: "It's sad because I think Khader has been neglected and wasted his life and the killers, we don't know who they are and how has been killed.
"What we looking for today is for justice and that justice must be served for Khader."
Mr Saleh was married with a young son.
"Khader has left as well a son and this young boy has lost his father today, he is fatherless," he added.
"He will know one day who murdered his father.
"But the reality is we feel unjustice (sic) - the way Khader has been treated in prison was absolutely shocking.
"I have heard that he has been beaten before his death and nothing has been done."
The arrested men, two aged 21, a 20-year-old and a 23-year-old, remain in police custody.