The group had congregated at the site of an earlier hit-and-run accident that had killed a resident of their neighborhood when they in turn were hit by the truck, Chacaj added.
Bodies of the victims of the accident in Nahualá.
Police commissioner Ramirez Feliciano said the truck had now been found.
Three of the dead were children, Chacaj said.
Another 20 people were injured in the accident, several seriously, and were being treated in hospitals.
The truck was apparently travelling without its headlights on.
It has been reported that an eight-year-old girl is among the deceased.
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According to a representative of the local fire service, at least 17 injured people have been taken to a hospital in Solola, while nine more injured people have been taken to a hospital in the city of Quetzaltenango. Photos showed bloodstains, shoes and other debris on the asphalt amid yellow police tape.
Guatemala's President Jimmy Morales said response was being coordinated to bring support to the relatives of the accident victims.
On Twitter, the Guatemalan government said it "deeply lamented" the tragedy.
"We are now coordinating actions to provide all the support to the victims' families".
Authorities set up a morgue in the local community hall where family members arrived to pick up the bodies of loved ones and take them back to their homes before funerals later Thursday.
Early reports had put the number of those killed at about 30, but later Chacaj said that emergency personnel had recovered 18 bodies.
Traffic rules are frequently ignored in Guatemala, a nation of some 16.5 million people.