A group of people had converged at the scene of another hit-and-run in which a local community leader was killed
A truck driver barrelled into a group of indigenous Guatemalans on Wednesday night, killing 18 before fleeing the scene as police began a manhunt.
The incident occurred at the entry point to the western town of Nahuala, which has a population of around 65,000, most of whom are K'iche' Maya.
A group of people had converged at the scene of another hit-and-run in which a local community leader was killed, when they in turn were hit by a truck with its lights off on a road without street lighting.
"In total there are 18 deaths that have been counted in this tragedy," fire department spokesman Cecilio Chacaj told AFP, revising down a previous toll of 30 that he attributed to early chaos on the scene where bodies were piled up on top of another.
Chacaj added the dead included three minors while 20 had been wounded, some seriously, and were being treated in hospitals.
The government announced three days of mourning from Thursday, while President Jimmy Morales tweeted: "We are currently coordinating actions to provide all the support to the victims' families. My condolences."
Chilling images broadcast on local television showed the victims' bodies in their brightly collared clothing laid out in a line on the asphalt, as relatives cried out in anguish.
Driver on the run
Police spokesman Pablo Castillo told reporters that an operation to capture the driver was now under way.
He added that investigators were initially prevented from getting to the scene by angry relatives but were given access after opening a dialogue with community leaders.
Local reports said the man killed in the first accident was a community leader and president of the state Council of Urban and Rural Development, though authorities have not confirmed this.
The semi-trailer did not have its lights on when it struck the group of people which may have been why the driver did not spot them, while the highway was also unlit, according to local media.
Traffic rules are frequently ignored in Guatemala, a Central American country of some 16.5 million people.