Police patrolling San Diego's schools have a new weapon at their disposal – a 20-ton army-surplus tank.

San Diego Unified School District police acquired the mine-resistant ambush protected vehicle, or MRAP, from the US government's military surplus program.

The vehicles, often referred to as tanks by American media, have been used in Afghanistan and Iraq. They are capable of smashing through barricades and can withstand bomb blasts.

Police departments in the US also use them for the deployment of Swat teams.

San Diego's MRAP is stored in a garage at a school in the southeast of the city, according to local broadcaster KPBS.

The program under which unneeded military weapons, armour and vehicles are supplied to local police forces at no cost came to national attention in the days following the police killing of Michael Brown in Ferguson, Missouri, last month.

Television audiences were "shocked" by images of combat-ready police officers in body armour backed up by armoured vehicles facing down peaceful protesters, the Daily Telegraph says.

Scott Barnett, a San Diego school district trustee, criticised the district police department for bringing in the military vehicle.

"My view, even before Ferguson, is that this is not something we should have in our fleet," he said. "If the district believes that we need additional capacity such as the armoured vehicle, then that should be discussed by the school board."

But district police chief Ruben Littlejohn defended his actions and turned on Barnett. "I don't report directly to the Board of Education, and I went through my chain of command as required," he said. "The fact that you weren't informed at that time was not something that I bothered myself with."

According to reports, police now plans to turn its MRAP into a rescue vehicle by painting it white and filling it with medical supplies.