Perez won over voters by pledging to crush criminality with an "iron fist." Mexican drug cartels have seized large swatches of territory in Guatemala, which has one of the world's highest homicide rates.
Perez, 61, is the first military officer elected as Guatemalan president since the end of a military government 25 years ago. He served in that administration as director of intelligence.
His tough campaign resonated in a country of more than 13 million people where murders are committed at a rate of 41 for every 100,000 residents, according to a recent U.N. homicide report. That is well over twice the rate in neighboring Mexico.
"He was the only one who from the start had a national security program," said Leonel Archila, a 43-year-old businessman among the 2,500 people at the ceremony. All wore blue or white shirts and sat in sections organized to form a giant Guatemalan flag.
The mood was marred by the assassination a day earlier of congressman Oscar Valentin Leal Caal, who was shot to death outside the headquarters of the former ruling party. Colleagues said he had been in negotiations to join the new president's party.
"We are profoundly concerned about the assassination of Leal, an elected representative of the people, and we hope to see the investigation generate results," U.S. Ambassador Arnold Chacon said.
Close advisers say Perez supports meeting the conditions set by the U.S. Congress for restoring aid it eliminated in 1978 — halfway through the Central American country's 36-year civil war.
Among the requirements is reform of a weak justice system that has failed to bring to justice those responsible for abuses during the conflict. A U.N.-sponsored postwar truth commission said state forces and related paramilitary groups committed most of the killings.
The U.S. also insists that the government back a U.N.-supported international anti-corruption team whose prosecution effort has been criticized by Guatemala's political elite.
Perez has long insisted there were no massacres, human rights violations or genocide in a conflict that killed 200,000 civilians, mostly Mayan Indians.