Peruvian President Pedro Pablo Kuczynski has pardoned former authoritarian leader Alberto Fujimori, triggering Christmas Eve street clashes as protesters denounced the decision as part of a crude political deal.

The decision clears Fujimori of convictions for human rights crimes and corruption when his right-wing government was in power from 1990 to 2000.

The move could define Mr Kuczynski's legacy and rewrite political alliances.

At least two cabinet ministers who objected to the pardon told Mr Kuczynski they wanted to resign.

Mr Kuczynski, a former Wall Street banker who vowed as a candidate not to pardon Fujimori, based his decision on a medical review that found Fujimori suffered from "a progressive, degenerative and incurable disease", according to a statement from the president's office.

Late last night, Fujimori was taken to hospital from prison by ambulance to treat a drop in blood pressure and abnormal heartbeat.

Many in Peru saw the pardon as part of a quid pro quo. Three days earlier, Fujimori's loyalists - led by his son Kenji - unexpectedly saved Mr Kuczynski from a vote in Congress that nearly removed him from office.

In a video Kenji Fujimori shared on social media, his gray-haired father, connected to tubes in hospital, was seen smiling after reading Mr Kuczynski's announcement of the pardon on a mobile phone.

"To save his own skin he cut a deal with Fujimori's supporters to infamously pardon a corrupt killer," said Veronika Mendoza, a leftist leader who competed against Mr Kuczynski in last year's presidential election.

Mr Kuczynski's centre-right government has repeatedly denied that a pardon for Fujimori was part of political negotiations.

The pardon may prompt one of the biggest political realignments in Peru since Fujimori fled to his parents' homeland of Japan in 2000 as a corruption scandal brought his decade in power to an end.

He was extradited back to Peru in 2007 and later found guilty of commanding death squads that massacred civilians, bribing lawmakers and having a hand in the kidnapping a journalist, among other crimes.

Despite his downfall, the right-wing populist movement that Fujimori built has remained one of the most potent political forces in Peru.