"They fought and some were hurt," but none accepted medical treatment, Carlos Salazar a spokesman for the firefighters, said.
Wendy Mendez, a member of the Children's Collective, said military police sprayed the demonstrators with tear gas and pepper spray.
"We believe that an institution that committed genocide should not be honored at the national level," she said.
A 1998 report by the Roman Catholic Church's human rights office blamed Guatemala's military for the overwhelming majority of atrocities committed during the civil war.