Two UN human rights experts on Thursday said the pardon of former Peruvian president Alberto Fujimori was a "slap in the face" to victims of his brutal rule.

The decision triggered protests by thousands of angry Peruvians denouncing both the pardon and current President Pedro Pablo Kuczynski, who decreed Fujimori's release on Sunday, three days after narrowly surviving impeachment on graft allegations thanks to Fujimori's son, Kenji.

Fujimori, 79, had been serving a 25-year prison sentence for the murder of 25 people by death squads and other human rights abuses committed during his time in office from 1990 to 2000.

"The presidential pardon granted to Alberto Fujimori on politically motivated grounds undermines the work of the Peruvian judiciary and the international community to achieve justice," UN special rapporteurs Agnes Callamard and Pablo de Greiff said in a statement Thursday.

"We are appalled by this decision. It is a slap in the face for the victims and witnesses whose tireless commitment brought him to justice," the experts said.

"It is also a major setback for the rule of law in Peru: a humanitarian pardon has been granted to someone convicted of serious crimes after a fair trial, whose guilt is not in question and who does not meet the legal requirements for a pardon."

Kuczynski said his pardon was given on humanitarian grounds, based on the former president's ill health.

Fujimori, who has been in a Lima clinic since Saturday, was transferred from prison after suffering low blood pressure and an irregular heartbeat.

As president, Fujimori earned respect from many Peruvians for his ruthless campaign to defeat leftist Shining Path guerrillas, but his brutal, illegal methods were also condemned by other parts of Peruvian society and foreign observers.

The timing of the pardon was seen by many in Peru as suspicious, coming days after Kuczynski barely survived an impeachment motion in the opposition-controlled Congress.

It was only because Kenji Fujimori convinced some members of a party led by his sister Keiko not to support the impeachment that Kuczynski was not removed from power.

"The government should not give in to political pressure and ignore its domestic and international obligations," the UN special rapporteurs said.

"Fujimori must be treated like any other person convicted of serious human rights crimes."

Kuczynski was questioned on Thursday by anti-corruption prosecutors over his alleged links to Odebrecht, a disgraced Brazilian construction firm that has admitted to bribing officials to secure public work contracts.