Care Centre general director Junji Kusamitsu speaks during a press conference in Yokohama on Wednesday. AFP

The man accused of murdering 19 disabled people at a Japanese care home in one of the country's worst ever mass killings pleaded not guilty Wednesday to the horrifying attack as his trial began.

Satoshi Uematsu, a former employee of the care centre outside Tokyo, did not dispute his involvement in the horrifying stabbing rampage during his first court appearance on charges including murder.

Satoshi Uematsu's lawyer leaves the Yokohama District Court in Yokohama on Wednesday. Charly Triballeau/AFP

After prosecutors read out the details of the charges, the judge asked Uematsu "is there anything in the charges that differs from the facts?"

"No there isn't," Uematsu replied, dressed in a navy suit with a white shirt and tie, his long black hair tied back in a ponytail.

But despite admitting the attack, Uematsu's lawyers entered a plea of not guilty, saying their client was suffering a "mental disorder" linked to his use of marijuana.

"He abused marijuana and suffered from mental illness. He became a different person and as a result, this incident happened," his lawyer said.

"He was in a condition in which either he had no capacity to take responsibility or such a capacity was significantly weakened."

The session was disrupted shortly after it began when court security restrained Uematsu after he reportedly attempted to put something in his mouth.

The disturbance prompted the judge to call an unscheduled recess, and Uematsu was not in court when the session briefly resumed in the afternoon.