The crackdown follows an attack on Friday in the city of Weifang in which a farmer broke into a school, attacking children and a teacher with a hammer before setting himself on fire and burning himself to death.

Friday's attack was the third such incident in three days, and the fifth in the space of just over a month.

Pupils wounded

On Wednesday, a 33-year-old former teacher broke into a primary school in the southern city of Leizhou, wounding 15 pupils and a teacher with a knife.

The attacker had reportedly been on sick leave from another school since 2006 for mental health problems.

A day later, a 47-year-old unemployed man armed with a 20-centimetre knife wounded 29 young children, as well as two teachers and a security guard at a school in the eastern city of Taixing.

Five of the injured children were reported to be in a serious condition in hospital.

Parents of children at the Taixing school have been protesting outside a city hospital demanding better security measures in response to the attacks.

The attacks have been given little coverage in Chinese state media, in what observers say may reflect a desire to avoid further copy cat attacks, or to avoid overshadowing blanket coverage of the opening of the showpiece Shanghai Expo, a pet government project.