A father has spoken out after three boys aged 11 and 12 have been arrested on suspicion of sexually assaulting other youngsters in one of the Bristol's most popular parks.

A fourth boy – aged just eight – is also said to have been involved in the alleged attack in Hengrove Park but is below the age of criminal responsibility, which means he cannot be prosecuted.

The incident happened on Saturday afternoon but details were only confirmed by Avon and Somerset police yesterday, after an account by the father of one of the alleged victims was published in a national newspaper.

The report in The Sun claimed the attackers, three brothers and a friend, forced their three victims, all aged 10, to drink urine and made one of them strip naked before forcing the others to perform sex acts on him. It was claimed the naked boy was then made to eat dog mess, with the attackers threatening to kill the victims unless they obeyed.

The father, who cannot be named for legal reasons, said he believed his son's attackers were copying an incident in Doncaster earlier this year, in which two boys were attacked and tortured by brothers aged 10 and 11. He said: ''It was a copycat attack. These brothers have caused havoc before. They live with their mum, who is a single parent, and doesn't seem to have any control over them."

The father said his son had become "withdrawn" since the incident and added: "What has happened is unforgivable."

Police have now appealed for anyone who was in the park on Saturday afternoon and believes they may have seen something which could help the inquiry to come forward.

Hengrove is home to one of the South West's most popular play parks, built at a cost of about £1.7 million and attracting over 100,000 visitors every year. It was not clear last night whether the incident took place near to the four-acre play area.

A force spokesman said: "We can confirm that three youths, aged 11, 12, and 12, have been arrested on suspicion of sexual assault and have been bailed pending further inquiries. A fourth youth was detained but then released. Three boys of primary school age were allegedly assaulted.

"This must have been very distressing for the victims involved. The park would have been very busy on Saturday afternoon."

Anyone with information can call the police on 0845 456 7000.