Three teachers at a nursery school in central Japan have been arrested on suspicion of routinely abusing toddlers, including hitting their heads, holding them upside down and locking them up in a bathroom, in a case that has triggered outrage and allegations of a cover-up.

The Shizuoka prefectural police said they arrested three women on Sunday on suspicion of assaulting at least three toddlers in June at a nursery school in the city of Susono at the foot of Mount Fuji.

Susono Mayor Harukaze Murata told reporters on Monday that he has also filed a criminal complaint against the school’s director, Toshihiko Sakurai, for allegedly covering up the abuses, and urged police to widen their investigation.

In one case in June, one of the teachers is accused of holding a boy upside down. Another teacher pushed a girl in the face, according to police, and the third slapped the head of another boy.

Their arrests followed a police search of the private school on Saturday in response to the city’s revelation last week of 15 counts of alleged abuses between June and August.

Results of an internal investigation revealed that the three teachers routinely abused toddlers in their care, including slapping their face and heads, forcing them to cry, threatening them with a cutter knife, verbally abusing the children by calling them “ugly” and “fat”, and locking them up inside a toilet or storage room, according to the city.

Mayor Mr Murata accused the school director of “covering up” by having other teachers sign a paper requiring them to conceal the problem and delaying giving an explanation to the parents, calling his handling of the issue “heinous”.

The three teachers, all in their 30s, reportedly told investigators that their treatment of the toddlers was “discipline”, while the director said he had the document signed only to protect the privacy of those involved and denied trying to cover up the abuses.

The city has faced public criticism for sitting on the case for more than three months since a whistleblower first came forward to reveal “inappropriate” cases at the school in mid-August.

Mr Murata said he takes the criticism seriously and will take a pay cut for two months while also punishing three senior officials.

Separate investigations into possible abuse have been taking place in two other nursery schools.

In Sendai, northern Japan, officials are carrying out an internal inquiry following allegations that children had to strip into their underwear during meal time so they do not dirty their clothes.

And at a school in another northern city, Toyama, police are investigating teachers on suspicion that they locked up crying children in a storage facility or poked their back with a stick to order them to move.

Experts say nursery teachers tend to be low paid and schools face chronic staff shortages and a harsh working environment.