The gunman, Wellington Menezes de Oliveira, shot dead 12 children and wounded 11 others - many in the head - and only stopped firing in the rampage at his old primary school in Rio de Janeiro when he was shot in the leg by a police officer and turned one of his revolvers on himself, officials said.

12 children were killed and 22 were injured in a deadly school shooting incident in Rio de Janeiro on Thursday.

Earlier, it was reported that Mr Menezes de Oliveira shot dead 10 children, but the toll was later clarified as 11 dead, before the death of a 13-year-old boy in hospital raised the toll to 12.

In one of the worst cases of its kind in Brazil's history, 23- year-old Wellington Menezes de Oliveira entered a public school in Rio's western region.

According to local police, the victims?all are aged between 12 and 14. And at least 18 injured teens are now receiving treatment at the local Albert Schweitzer Hospital.

Police said the shooter was a graduate of the school. A city education official said the gunman came to the school one year ago, indicating the shooting was a premeditated crime.

Menezes' motives remained unclear. According to a police spokesman, the shooter left a note, in which he stated that he was HIV-positive and believed "kids are impure."

He entered the Tasso da Silveira school in the neighborhood of Realengo around 8 a.m. local time, saying he would give a lecture to the 8th graders. Then, he reportedly entered the classroom and started shooting randomly. One of the students managed to escape and called the police.

Menezes tried to escape, but he was intercepted by a police officer, who ordered him to drop his two revolvers.

As the killer refused to do so, the officer shot Menezes in the leg. After being injured, according to the police, Menezes shot himself dead.

Brazil's President Dilma Rousseff, who learned the news at a public meeting in Brasilia, said: "This sort of crime did not use to happen in Brazil. I think that everyone here, men and women, is joined in repudiation to this act of violence against defenseless children."