MEXICO CITY (Reuters) - Nicaraguan police have used lethal force against protesters, aiming for heads, necks and chests, Amnesty International said on Tuesday in the second of two international reports condemning President Daniel Ortega’s response to dissent.

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At least 81 people have been killed and 868 wounded since April 18, according to the rights group, in violence “characterized by the excessive use of force, extrajudicial executions, control of the media and the use of pro-government armed groups.”

More than a month after changes to the Central American country’s social security system triggered student-led protests, demonstrations have morphed into a daily challenge to the rule of Ortega, a former Marxist guerrilla.

Published after days of resurgent violence, the report adds five to the preliminary death count announced last week by the Inter-American Commission of Human Rights following an investigation sourced in hundreds of complaints.

“The strategy for repression appears to have been directed from the highest levels of government,” Amnesty International said in a statement. “President Daniel Ortega and Vice-President Rosario Murillo repeatedly demonized demonstrators and denied they were being killed.”

The government acknowledged a request for comment on the Amnesty report but did not immediately respond. Earlier this month, Ortega lamented the violence and said there had been deaths on all sides.

Ortega, an ally of socialist Venezuela, has resisted protesters’ demands that he resign over the killings. His government rejected a suggestion by the Organization of American States that it call early elections.

In a round of “national talks” with members of civil society last week, government representatives acknowledged the recommendations of the Inter-American Commission’s report but issued no further official statements.

Clashes this week included an armed attack apparently by pro-government groups on a university campus and witnessed by Amnesty International’s America’s director Erika Guevara-Rosa.

The report, based on a visit to Nicaragua in early May and delivered by Nicaraguan rights activist Bianca Jagger, also documented irregularities in the way authorities investigated deaths, including the cases of two people whose families were forced to sign waivers in hospital denying them a right to autopsies as a condition of receiving official death certificates.