Malaysia's constitutional monarch, Sultan Muhammad V, administered the oath of office just before 10 p.m. (1400 GMT), in a ceremony carried live on state television from the palace.

Mahathir, 92, dubbed the "Father of Modern Malaysia" during his previous 22 years in power until 2003, was dressed in a traditional black "baju melayu" tunic and sarong, with an Islamic cap on his head. His return from retirement makes him the oldest elected leader in the world.

Hundreds of Malaysians lined the road leading to the palace, waving party flags and cheering. The Election Commission announced the result long before dawn and there was some consternation in the capital Kuala Lumpur over the time taken to swear in the new prime minister.

At a press conference after being sworn in, Mahathir reassured the financial community and said he would prioritize stabilizing the economy and return billions of dollars lost in a graft scandal at state fund 1Malaysia Development Berhad (1MDB).

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"We believe that we can get most of the 1MDB money back we have to increase the confidence of investors in the administration," he told reporters.

Malaysian markets were closed and will reopen only on Monday, but overseas investors were nervous about the ouster of Prime Minister Najib Razak after a decade in office and the ringgit lost four percent in offshore trading.

An overseas Malaysian equity fund initially showed a 6 percent drop in share values but partly rebounded on Thursday.

"There should be no cause for any devaluation of the ringgit," Mahathir said.

"As you know, we cannot revalue the ringgit too much, or else we will not be competitive, but we will try to make the ringgit as steady as possible," he added.

Mahathir was known for his strongarm, sometimes pugnacious data-style of rule intolerant of dissent from 1981 to 2003, but also for transforming his Southeast Asian country from a sleepy backwater into a modern industrialized nation.

He came out of retirement to take on his ex-protege Najib.

Mahathir's alliance of four parties trounced Najib's Barisan Nasional (BN), the first time it had ever lost an election.

Earlier on Thursday, Najib appeared to raise doubts that Mahathir would immediately take office because no single party had won a simple majority of seats in the 222-member parliament, and it would be up to the monarch to decide.

Official results showed that Mahathir's coalition won 121 seats, comfortably more than the 112 required to rule. But it has not been formally registered as an alliance.

In a jubilant mood, Mahathir joked about his age and about being labeled a dictator at the news conference.

"Please remember, I am a dictator," he joked.

REUTERS