The boy, Javier Morena, was the oldest son of a poor family who sold fruit in the tough district of Iztapalapa.
The abduction proved that the country's plague of kidnappings afflicts the working class as well as the wealthy.
Javier disappeared while playing at the market on Sunday 26 October, Mexico City authorities said on Monday.The boy's family spent days looking for him, finally persuading a local television station to post his picture on the news three days later.
A taxi driver recognized the boy, and went to the market to find the family. He told them that he had given the boy and a teenager a ride from the market to nearby Mexico state, and the teenager had told him the boy was crying because his younger brother had been stolen.
The driver dropped the two off a block from the police station, and the teen told him they were meeting the boy's mother there.
The family showed the driver a picture of their son. Also in the picture was a 17-year-old family friend, who the driver recognised as the alleged kidnapper.
The police raided the 17-year-old's home, and he and his family and two others confessed to having killed the boy before they could ask for a 300,000-peso (£15,000) ransom, Mexico City Attorney General Miguel Mancera said in a statement.Mancera said the assailants injected the boy with acid and buried him on a hill outside the capital.
Five people, including the 17-year-old, are under arrest. It was unclear if the group had carried out other kidnappings.
Javier was buried early yesterday. Hours later, sitting in her home of cinderblock and corrugated tin, the boy's mother, Laura Vega, said she has no idea why the kidnappers targeted her family
But she said she felt they should face the death penalty, long banned in Mexico, and that they should 'suffer the way my son suffered.'
'He didn't have to die like that, far from his parents,' she said, her eyes red and swollen from crying.
'If he had to go to God, it shouldn't have been like that.'
The child's death recalled the recent kidnapping and slaying of Fernando Marti, the 14-year-old son of a sporting goods magnate whose death prompted a national outcry against crime.Young Marti's decomposing body was found in the trunk of a car even though his family reportedly paid a ransom. Prosecutors said a federal lawman was part of the gang that kidnapped Marti.
Outrage over that case prompted more than 100,000 people to march through Mexico City in August to demand an end to endemic police corruption and rising crime.
Kidnappings in Mexico have increased by nine per cent since last year.
The attorney-general's office blames drug cartels, police officers and informants who point out potentially lucrative victims.
