BARCELONA (AFP) - A 13-year-old boy armed with a crossbow and knife who killed a teacher at his school in Spain "burst into tears" after he realised what he had done, the gym teacher who subdued him said on Tuesday.

David Jurado described in an interview with television station Telecinco how he managed to calm the boy, who also injured two other teachers and two students at a Barcelona secondary school on Monday.

The teacher said he was in the school courtyard when he saw students running and crying.

One of the youngest students told him there was a "wacko" who was "threatening to kill everyone".

Jurado said he then raced to the second floor where he saw the boy with a knife in his hand. The teacher said he went looking for a stick and when he returned he saw the boy inside a classroom preparing a molotov cocktail.

"My plan was to grab him from behind and hit his hands to disarm him," Jurado said.

"I started to talk to him, there's no need to get into details, but I tried to make him snap out of it because he wasn't himself, he was acting like a robot. At first he'd repeat things mechanically but gradually he became himself".

"He dropped his weapons, I dropped the stick, he slowly returned to reason. We sat down to talk and he collapsed, he burst into tears. I think the world collapsed for him," Jurado added.

He described the boy as a "fantastic student" before Monday's attack who "behaved wonderfully" in the classes he taught.

Schools across Spain's northeastern Catalonia region observed a minute of silence on Tuesday in memory of the 35-year-old teacher who was killed in the attack.

It was the first time that a teacher has been killed at a school in Spain since 1975.

The lives of four people injured in the attack are not in danger.

The boy who carried out the attack was admitted to the psychiatric unit of a Barcelona hospital.

Under Spanish law children below the age of 14 are not held legally responsible for crimes and cannot be jailed.