A former interior minister was sworn in as Malaysia's premier today, marking the return of a scandal-mired party to power after a reformist government's collapse but ex-leader Mahathir Mohamad, 94, slammed the move as illegal.

The Southeast Asian nation was plunged into turmoil after Mahathir's reformist "Pact of Hope" alliance, which stormed to a historic victory in 2018, collapsed amid bitter infighting.

Mahathir -- world's oldest leader -- initially quit as premier but then sought to return. He lost out, however, to little-known Muhyiddin Yassin.

The king's decision Saturday to pick Muhyiddin as premier was greeted with shock as Mahathir's allies claimed to have enough support, and sparked widespread anger that the democratically elected government had been ejected.

Just before Muhyiddin's inauguration, Mahathir accused him of betrayal and said he would seek a parliament vote challenging the new premier's support -- signalling the political crisis is far from over.

Muhyiddin's coalition includes the United Malays National Organisation (Umno), the party of disgraced ex-leader Najib Razak, as well as a hardline group that wants tougher Islamic laws.

Umno was the corruption-riddled lynchpin of a coalition thrown out at the 2018 elections amid allegations Najib and his cronies looted billions of dollars from state fund 1MDB. Najib is now on trial for corruption.

Despite a last-minute bid by Mahathir to prove that he had enough support to return as premier, Muhyiddin's inauguration went ahead today morning at the national palace.

Mahathir's alliance boycotted the ceremony.

To become premier, a candidate must prove to the king, who appoints the prime minister, that he has the backing of at least 112 MPs.

Meanwhile public anger was growing at the ejection of the reformist alliance, with the hashtag "NotMyPM" trending on Twitter and more than 100,000 people signing a petition that said the move was a "betrayal" of voters' choice at the 2018 poll.

Mahathir -- who served a first stint as premier from 1981 to 2003 before staging a comeback two years ago -- also raised concerns that ongoing court cases against Najib related to the 1MDB scandal could be affected by the change of government.

Analysts said Muhyiddin's government was in a weak position as it was not clear whether he had a parliamentary majority, and was a member of a tiny party without a strong support base.

He is a Muslim nationalist -- he once controversially described himself as "Malay first" and Malaysian second -- and there are concerns his leadership could worsen already tense race and religious relations in the multi-ethnic country.

About 60 percent of Malaysia's population are ethnic Malay Muslims and the country is also home to substantial ethnic Chinese and ethnic Indian minorities.