Police in riot gear fired tear gas at the crowd in the highland town of Santa Cruz Quiche on Wednesday after a confrontation with stone-throwing protesters.
"There was a confrontation between the police and a group of youths. The police fired tear gas bombs to disperse them. The protest continues but is now peaceful," human rights official Raul Rodriguez said.
The rally follows a week of fierce clashes between police and protesters angry at the US - Central American Free Trade Agreement, or Cafta, approved by the Guatemalan Congress on Thursday.
On Tuesday, a teacher was killed and several people seriously injured when hundreds of police and soldiers tried to disperse a demonstration in the Huehuetenango region, close to Guatemala's border with Mexico.
Witnesses say the security forces used live ammunition in an attempt to reopen a stretch of the Pan-American highway blocked by the protest.
Critics of the trade pact say the
poor will be adversely affected
The highway remained closed on Wednesday as teachers flocked to the site of the killing, forcing police to retreat from the area to avoid further clashes.
The government of Guatemalan President Oscar Berger has used force on several occasions to break up protests since it came to office in January 2004. In August, 10 people were killed during a police-led eviction of a squatted farm.
Central American governments hope the contentious Cafta will lower or eliminate tariffs on their exports.
Poor people affected
Critics in Central America say patent rules in the pact will limit poor people's access to lifesaving drugs, and many worry that small-scale farmers will be unable to compete against subsidised US agriculture.
Guatemala, Honduras and El Salvador have approved the pact, considered the most explosive trade issue before the US Congress this year.
In the United States, Cafta is languishing in Congress while the Bush administration tries to round up support for it.