Otto Perez Molina has been sworn in as Guatemala's president, calling on the US and Mexico to help him fight a wave of drug trafficking and violence that has overwhelmed the Central American country.

"The change has begun," Perez told cheering supporters on Saturday in Guatemala City.

"We are committed to the peace and integral security that we all desire.

"Today I call on my international associates to combat drug trafficking: Mexico, Central America, and I make a special call to the United States" Perez said.

The new president, 61-years-old, is the first former 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.

Mexican drug cartels have seized large swatches of territory in Guatemala, which has one of the world's highest homicide rates.

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 annually, according to a recent UN homicide report.

That is well over twice the rate in neighbouring Mexico.

"He was the only one who from the start had a national security programme," said Leonel Archila, a 43-year-old businessman among the 5,000 people at the ceremony.

About half were Guatemalans bused in from around the country.

All wore blue or white shirts and sat in sections organised to form the shape of a giant Guatemalan flag.

Marred inauguration

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," US Ambassador Arnold Chacon said.

Close advisers say Perez supports meeting the conditions set by the US 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 UN-sponsored post-war truth commission said state forces and related paramilitary groups committed most of the killings.

Perez has long insisted there were no massacres, human rights violations or genocide in the conflict that killed 200,000 civilians, mostly Mayan Indians.

"I suffered and lived through the armed conflict and 15 years after having signed the (peace) agreement Guatemalans are still being betrayed," Perez said on Saturday.

"I pray to God for a true reconciliation. I pray that my generation is the last of war and the next one is the first of peace."