Spain was rocked this week by news an 11-year-old girl had allegedly been gang-raped by minors near Barcelona, sparking fierce debate about how to halt growing violence among youngsters.

Catalan television said the attack occurred in November at a shopping centre in Badalona, with the victim forced at knifepoint to enter the toilets by a group of six minors who then raped her.

Questioned by AFP, the Mossos d'Esquadra regional police confirmed they had received a complaint in December about a "sexual assault" -- which covers a range of offences, including rape -- involving minors.

But they refused to give further details, citing the age of those involved.

And prosecutors said they could not confirm "any information about the lawsuit because the inquiry was being conducted behind closed doors".

The local authorities in Badalona, a seaside town of 223,000 residents 10 kilometres (six miles) north of Barcelona, expressed "willingness" to join the prosecution in any future trial "to defend the victim of the group sexual assault".

Under Spanish law, any third party can join a prosecution, even if they have suffered no direct harm.

However, with media reports saying most of the alleged attackers were under 14 -- the legal age for criminal responsibility in Spain -- few are likely to be brought to trial.

Several weeks ago, the juvenile prosecutor's office requested that both of the eldest, who are 14, be held in a special centre, but the judge agreed only in one case, with the other allowed out on probation, El Pais newspaper said.

- 'We have a problem' -
Media reports said the victim's brother received anonymous threatening messages after the family filed a complaint with the police, which managed to track down the person who sent the messages.

On Friday, the juvenile section of Catalonia's top court confirmed sending the alleged sender of the messages, a minor, to a closed facility for six months detention "for obstruction of justice and threats in connection with a lawsuit over the alleged group sexual assault of a minor in Badalona".

The case has raised widespread alarm in Spain over the growing number of sexual assaults by minors.

"As a society, we have a problem and we urgently need to address the emotional and sexual education of these minors," regional police spokeswoman Montserrat Escude told reporters.

The situation requires "that we all understand and examine what is happening to us as a society, because these behaviours are intolerable yet they have become normalised", she said.

Figures released by the Catalan police show that last year, 12 percent of sexual crimes in this wealthy northeastern region of Spain were committed by minors.

"As the offenders are becoming younger, so should we step up sex education for adolescents," an editorial in La Vanguardia, Barcelona's main newspaper, said Friday, pointing to pornography as a key concern with regards to teenage sexual violence.

Last year, police investigated 1,023 minors under the age of 14 for various offences, of which around 100 involved a sexual offence, the paper said.