The New York-based NGO claimed there had been political meddling in Anwar's Sodomy II court case, citing allegations that lead prosecutor Muhammad Shafee Abdullah had been paid RM9.5 million by Prime Minister Najib Abdul Razak.
"We respectfully urge Your Majesty to recognise the undeniable political meddling in Anwar's case, order him to be released from prison, and clear him of his conviction.
"These seriously questionable payments to Shafee from Najib, compounded with the fundamental lack of evidence in support of Anwar's conviction, show that Anwar's political opposition aimed to strip him of his political standing and damage his reputation permanently," HRF wrote in the letter dated yesterday.
They noted that Shafee has yet to deny the allegations against him.
The letter was signed by HRF president Thor Halvorssen, and chairperson Garry Kasparov.
Putrajaya, in a surprise move on July 22, 2013, recruited Shafee to lead the prosecution team against Anwar at the Court of Appeal instead of relying on public prosecutors as per conventional practice.
The Court of Appeal on March 7, 2014, overturned the High Court of Kuala Lumpur's decision to acquit Anwar, sentencing him to five years in prison.
The Federal Court on Feb 10, 2015 upheld Anwar’s conviction for sodomising his former aid Saiful Bukhari Azlan.
Whistleblower site Sarawak Report had previously alleged that Shafee received RM9.5 million from Najib in two tranches on Sept 11, 2013 and on Feb 17, 2014.
Shafee on June 9 said he would make a statement on the matter but has since remained silent.
The government had told Parliament that Shafee was paid a fee of RM1,000 to act as the lead prosecutor in the Anwar case.