The assault, which occurred on the seventh anniversary of a mass stabbing at a Japanese elementary school, was the latest in a series of knife attacks that have stoked fears of rising violent crime in Japan.
A 25-year-old man, Tomohiro Kato, was arrested with blood on his face.
"The suspect told police that he came to Akihabara to kill people," said Jiro Akaogi, a spokesman for the Tokyo Metropolitan Police Department.
"He said he was tired of life. He said he was sick of everything," Akaogi said.
The violence began when the man crashed a rented, two-ton truck into pedestrians. He then jumped out and began stabbing the people he had knocked down with the truck before turning on horrified onlookers, police said.
Police confirmed seven deaths - six men and one woman - but they could not say whether the victims had died of injuries from the truck or were stabbed to death.
Reports said the attacker grunted and roared as he slashed and stabbed at Sunday shoppers crowding a street lined with huge stores packed with the latest in computers, electronics, videos and games.
"He was screaming as he was stabbing people at random," a male witness told national broadcaster NHK.
Another witness told NHK that the suspect dropped his knife after police threatened to shoot him. Amateur video filmed by mobile phone showed policemen overpowering the bespectacled, bloodied suspect.
The attack paralyzed the district known as Electric Town and sent thousands of Sunday shoppers into a panic. Amateur video taken five minutes after the rampage showed shoppers helping victims and a man screaming, "Ambulance, Ambulance!"
At least 17 ambulances rushed to the scene, and rescue workers feverishly tended to victims in the blood-pooled street.
As night fell on Akihabara, several pedestrians stopped by and prayed at the crime scene. A bouquet of flowers, bottles of green tea and incense sticks were placed at the site.
Japan boasts a low crime rate compared to other industrialized nations and Tokyo, with a population of 12.7 million, is considered relatively safe. But stabbings, once rare in the country, have become more frequent in recent years.
In March, one person was stabbed to death and at least seven others were hurt by a man who went on a slashing spree with two knives outside a shopping mall in eastern Japan. In January, a 16-year-old boy attacked five people in a shopping area, injuring two of them.
A spate of knife attacks also have occurred in schools, the worst on June 8, 2001 when a man with a history of mental illness burst into elementary school near Osaka killing eight children. He was executed in 2004.