Anwar's offer marked a seismic shift in Malaysia's political landscape, as the vendetta between the two has seethed since Mahathir sacked Anwar as deputy prime minister in 1998 and had him jailed on sodomy charges that many observers said were politically motivated.
Jailed Anwar backs old foe Mahathir to bring down PM
Malaysia's "Game of Thrones" heated up on Friday, as the country's jailed opposition leader Anwar Ibrahim offered support to his old enemy, former premier Mahathir Mohamad, in efforts to oust the current prime minister, Najib Razak.
A year ago, Anwar was jailed again on sodomy charges, that he said were concocted by Najib's government to eliminate the threat he posed to its grip on power.
In a statement issued from jail on Thursday, Anwar said he would "support the position" of those in civil society, political parties and individuals "including Tun Dr Mahathir" in the push to remove Najib.
"He (Najib) is responsible for continuing to engage in selfish political acts, wreaking havoc upon administrative institutions, and burdening the Rakyat (people) with continued economic crisis," Anwar said.
Najib has faced sustained pressure to resign since the middle of last year over his links to a financial scandal involving state-owned fund 1Malaysia Development Berhad and deposits into his private accounts that the Wall Street Journal reported in mid-2015 as totaling almost $700 million.
The newspaper last week reported that it had traced more deposits, raising the total to over $1 billion.
Najib has maintained that he has not used the funds for personal gain, and has since been cleared of any criminal offence or corruption.
With Anwar behind bars, Mahathir has been Najib's harshest critic. Last week, Mahathir quit the United Malays National Organisation (UMNO), the party that has led every ruling coalition since Malaysia's independence in 1957, saying it had become Najib's party and he didn't want to be associated with corruption.
Najib's banker brother, Nazir Razak, has likened the political drama currently gripping Malaysia to HBO television's medieval fantasy saga "Game of Thrones".
Anwar was the ruling party's rising star in the late 1990s and was once thought to be Mahathir's intended successor, until they fell out over how to steer Malaysia out of the Asian Financial Crisis in 1997-98.
The former deputy prime minister has since waded through a deluge of legal problems, having spent a few years behind bars for the sodomy convictions and a corruption charge.
He was considered the government's biggest threat as the head of a three-party opposition coalition, that won the popular vote while losing a disputed election in 2013.
But last year, he was sent back to jail for five years after the country's apex court upheld a second sodomy conviction.
Nurul Izzah, Anwar's eldest child, said in a separate statement that the shared "zeal" to remove Najib does not absolve Mahathir of "his past abuses of power and wrongdoings, including huge economic miscalculations".
Mahathir is scheduled to hold a ceremony on Friday afternoon to bring together what he described as a "core group" of people to sign a memorandum of understanding on how they will work together to topple the prime minister.
Malaysians will be waiting to see if Mahathir accepts Anwar's olive branch.
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