Malaysia's Mahathir Mohamad will be sworn in as the world's oldest elected leader after his opposition alliance pulled off a stunning election win, ending six decades of rule by a coalition he once led.

He will not be sworn in as Malaysia's next prime minister on Thursday, a palace spokesman told Reuters, without giving a reason. Mahathir previously said he would be sworn in on Thursday.

Malaysians celebrated the 92-year-old's unexpected victory over Prime Minister Najib Razak, whose popularity had plunged over rising living costs and in the wake of a multi-billion-dollar graft scandal at 1Malaysia Development Berhad (1MDB).

Mahathir led the Southeast Asian nation for 22 years and his unexpected return to the prime ministership ends the previously unbroken rule of Barisan Nasional (BN), the coalition that had

governed Malaysia since independence from Britain in 1957.

"We are not seeking revenge ... what we want is to restore the rule of law," Mahathir said of Najib's scandal-plagued rule.

Mahathir appeared jubilant and sprightly at a news conference claiming victory overnight, even joking with reporters, and will have an audience with Malaysia's king later on Thursday.

The king will sign his letter of appointment as prime minister of Malaysia's constitutional monarchy during a ceremony at the royal palace in the capital, Kuala Lumpur.

Najib is also expected to address the media. He has not spoken publicly since the results were declared but a member of his Cabinet said they would accept the will of the people.

The stunning election outcome was expected to ruffle financial markets that were expecting a comfortable win for Najib and the BN.

Malaysia's currency weakened in offshore trading on Thursday, with the ringgit one-month non-deliverable forward falling 1.7 percent. The U.S.-traded iShares MSCI Malaysia ETF fell 6 percent.

The national stock market will be closed on Thursday and Friday after Mahathir declared a public holiday, but the ringgit currency weakened in offshore trading.