At least 19 people have been killed and about 20 wounded in a knife attack at a facility for the handicapped in a city just outside Tokyo in the worst mass killing in generations in Japan.

Police say they responded to a call about 2.30am (03.30 AEST) on Tuesday from an employee saying something horrible was happening at the facility in the city of Sagamihara, 50 kilometres west of Tokyo.

A man turned himself in at a police station about two hours later, police in Sagamihara said.