His parents found him sobbing so hard he couldn't speak, a 14-year-old locked up in an adult cell in Glasgow's most secure police station.

The boy - known only as Child X - was to spend more than two days in custody in what Scotland's new police complaints commissioner yesterday, in his first official pronouncement, ruled was a clear breach of guidelines.

His mother thinks she knows why he was held for so long.

"The police beat him," she claimed last night. "They kept him in because they wanted to wait until the bruises faded."

Child X was arrested at 2.30 on the morning of September 2, last year, drunk and carrying a kitchen knife he had found lying in the street a few moments before.

He was booked into Govan's high-security Helen Street police station with no record or log of any injury. The duty inspector later said he was "unco-operative, provocative and abusive".

His parents only got to see him at 10.30am. They found him bloodied and bruised, exhausted and terrified.

"When they opened the door to the cell all we could see was a wee boy breaking his heart," Child X's mother, a 47-year-old Glasgow teacher, said. "He could hardly speak for the sobbing. His jersey was all pulled out of shape. I could see his injuries from 7ft away. He said: Mum, I am so sorry. They gave me a doing'."

The boy's parents, who had spent much of the night desperately ringing around hospitals and searching for the boy, were confused.

"They had expected to come to the station to give their child a row. Instead, they found him shattered. His mother tried to take pictures of the injuries.

"They physically stopped me," she claimed last night.

The police eventually photographed Child X. The Herald has obtained those photographs. They clearly show his wrists marked red by ill-fitting handcuffs, a cut on the back of his head and bruises and scratches on his chest and shoulders.

Where and how was he hurt? No police officer has been prosecuted and Strathclyde Police last night declined to comment on the details of the case. Child X, however, has given his own version of events.

According to witness statements seen by The Herald, the boy was injured when a police officer stripped him of his trousers. "The officer ... put his left food on my chest under the left shoulder and collarbone and pressed down quite hard, but hard enough so that I couldn't sit back up. During that time the other officer pulled my trousers off."

Yesterday, Scotland's Police Complaints Commissioner, Jim Martin, ruled that Child X has been held in custody - for 59 hours - against the guidelines of Strathclyde Police and that the delay in informing his parents, which was three hours, had been "unacceptable".

Mr Martin told Strathclyde Police to apologise to the boy and his family and warned that the force's failures in respect of Child X's detention were "systemic".

The Herald understands, from highly credible sources, that X may not be the the only child detained in breach of Strathclyde's own standard operating procedures.

Mr Martin said: "It is of significant concern that in considering the overall complaint Strathclyde Police failed to identify and take steps to correct what appears to be a systemic failure in relation to the detention in custody of children.

"It is clear to me that various Strathclyde Police officers of various levels of seniority, were operating with an incorrect understanding of the force's standard operating procedures relating to the detention in custody of children."

The boy's mother believes other parents simply didn't know how to complain. She did - and she kept up pressure for 13 months.

She feels the police fobbed her off, until Mr Martin spent four months looking into the matter and upheld or partially upheld many of her complaints (although the allegations of police brutality fall outwith his remit).

"I don't think the police should be allowed to investigate themselves ever again," she said yesterday.

The force, meanwhile, will also have to reconsider how it detains children: Mr Martin has called on Her Majesty's Chief Inspector of Constabulary to look into overall policy.

Child X's mother has no problem with her son being pulled up by the police. He was arrested on the opening day of a major crackdown on knife crime after "stupidly" being lured away at night by an older boy and the promise of half a bottle of Buckfast.

She said: "My son did something wrong. My complaint is not that he has been arrested. I would have been really upset had they not stopped a 14-year-old who was out at two in the morning and drunk."

The boy was reported to the children's panel but has not been subjected to a supervision order because, his mother said, he came from a stable family.

Strathclyde were last night still not saying much about the case, saying they were still digesting the contents of Mr Martin's report.

A spokeswoman, however, said: "Although we cannot comment on the details of this individual case, we would wish to reassure the public that all decisions and actions taken by Strathclyde Police were on the grounds of the youth's safety and welfare and the safety of the general public.

"The family can be reassured that we take the findings of the commissioner very seriously."

Child X's mother and Mr Martin were yesterday both given the support of Kathleen Marshall, Scottish Commissioner for Children and Young People.

She said: "The UN Convention on the Rights of the Child says children should be detained only as a last resort and for the shortest appropriate period of time.

"I am heartened by the news that Her Majesty's Chief Inspector of Constabulary is to undertake a thematic review of the detention of children in custody. I will be contacting the inspector to express my own concerns about the detention of children."