Television footage showed green plastic sheets in the middle of an intersection set up to give privacy to the injured as they were treated.
Crowds surrounded the area and looked on after the knifing spree in Akihabara, which was closed to cars on the Sunday afternoon.
'Tired of life'
"I am shaking," a middle-aged woman told the NHK.
"It's pretty shocking, considering that I come here all the time," one middle-aged man said.
Police said they arrested Tomohiro Kato, 25, on the spot.
Tomohiri told NHK that he was "tired of life".
Private broadcaster Nippon Television showed footage of a slight, blood-splattered Tomohiro, being arrested by police.
Akihabara is crowded at weekend with both tourists and locals seeking cheap electronic gadgets.
Jiro Akagi, a police spokesman, confrimed Tomohiro's arrest and identity, but declined to give further details.