(CNN) -- At least seven people were killed in southwest Guatemala when security forces clashed with protesters blocking a highway, a hospital official said.
More than 30 others were hospitalized with injuries, 15 of them in serious condition, Damaris Juarez, a spokeswoman at Jose Felipe Flores Hospital, said Thursday.
Most of the victims suffered bullet wounds, she said, although the government says its forces were unarmed in the confrontation, which took place in the country's Totonicapan department.
Protesters from 48 local communities had gathered Thursday at a stretch of highway known as Cuatro Caminos to express their opposition to high energy prices, among other issues.
The leader of the protests, Carmen Tacan, blamed security forces for the deaths and demanded an investigation, according to a government news release.
Guatemalan President Otto Perez Molina ordered an investigation but suggested that the shooting originated from the protesters.
"We are willing to make available the weapons that they may have been carrying, although they -- the soldiers -- had no weapons, but were equipped with tear gas," Perez Molina said. "We have information that it may have been the civilians who initiated the shooting and upon hearing this, the contingent of soldiers exited their vehicles to disperse and avoid harm."
The president said he ordered police and military forces to retreat from the area where the protesters were.
In addition to high energy costs, the protesters also oppose an educational reform that will increase the duration of studies needed for a teaching degree from three years to five years.
CNN's Nelson Quinones contributed to this report.