Mr Mahathir was prime minister, at the head of the BN coalition, for 22 years, from 1981 until he stepped down in 2003.

Under his leadership, Malaysia became one of the Asian tigers - the group of countries which saw their economies expand rapidly in the 1990s. But he was an authoritarian figure who used controversial security laws to lock up his political opponents.

Most infamously his deputy prime minister, Anwar Ibrahim, was sacked and accused of corruption and sodomy - and later jailed on the latter charge - when he called for economic and political reforms.

Mr Mahathir was also a mentor to Mr Najib, who became prime minister in 2008.

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But Mr Najib has been dogged by corruption allegations. He has been accused of pocketing some $700m from the 1Malaysian Development Berhad, a state investment fund he set up.

He has vehemently denied all allegations and been cleared by Malaysian authorities but the fund is still being investigated by several countries. Mr Najib has been accused of stifling Malaysian investigations by removing key officials from their post.

Those allegations led to Mr Mahathir's surprise defection in 2016 from Barisan Nasional to join the Pakatan Harapan, saying he was "embarrassed" to be associated "with a party that is seen as supporting corruption".

Then in January, he said he would run for the leadership again.

But despite his historic win, uncertainty hangs over his tenure. Prior to his win, he intended to govern for two years before stepping down. He promised he would arrange a pardon for Anwar, clearing the way for him to be re-elected and take the top job.

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'Mountain of challenges'

Analysis: Jonathan Head, BBC South East Asia correspondent

This morning Malaysia has woken to an entirely new situation, the first transfer of power in its history, albeit to a very familiar leader. But there are huge unknowns. How willingly will Barisan Nasional, the coalition which has, in various forms, run the country since independence and embedded itself into all areas of governance, relinquish power?