ASUNCION (Reuters) - A U.N. official on Tuesday called for an investigation into the killing of a Paraguayan protester during violent demonstrations last week, while President Horacio Cartes canceled an overseas trip to concentrate on resolving the country’s political crisis.

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Amerigo Incalcaterra, Representative for South America of the U.N Office of the High Commissioner for Human Rights, said Paraguayan authorities were “doing what they have to do” after police stormed an opposition party’s headquarters last week and shot dead 25-year-old protester Rodrigo Quintana.

Cartes fired Paraguay’s national police chief on Saturday, and an officer was charged with homicide of the protestor on Monday. Government officials told a news conference on Tuesday there had been no order to open fire.

The incident occurred during unrest over an unpopular closed-door Senate vote to amend the constitution to allow for presidential re-election. Thousands took to the streets in protest, and a group of demonstrators stormed and set fire to the country’s Congress building.

“We expect the events will be investigated and that those responsible both for the acts of violence and for the death of a Paraguayan citizen will be punished,” Incalcaterra said after meeting with Paraguay’s foreign minister Eladio Lozaiga in the capital city of Asuncion.

Cartes canceled a trip to Argentina to attend a dialogue opposition leaders, currently set for Wednesday, Lozaiga told reporters on Tuesday. Cartes had invited opponents to the dialogue late on Sunday.

Paraguay’s Senate voted on Friday during a special session in a closed office rather than on the Senate floor. Twenty-five lawmakers voted for the measure, two more than the 23 required for passage in the 45-member upper chamber.

The measure to change the constitution to allow presidential re-election still would have to be approved by the lower house and via popular referendum.

The measure has strong support in the lower house, though the chamber’s leader has said it would not be considered while the dialogue is ongoing. The opposition Liberal Party has pledged not to attend the negotiations until the proposal was withdrawn.

Supporters of Cartes, a former soft-drink and tobacco businessman, want him to be able to seek a second term in a country that constitutionally forbid re-election after a 35-year dictatorship fell in 1989.