An alliance of opposition parties spearheaded by Mahathir Mohamad won Malaysia's general election, setting the veteran strongman on course for a return to the Prime Minister's office he occupied for 22 years.
Mahathir's stunning defeat of the coalition that has ruled the Southeast Asian country since independence from Britain six decades ago means that, at the age of 92, he will become the oldest elected leader in the world.
Official results showed that Mahathir's Pakatan Harapan (Alliance of Hope) had won 113 of parliament's 222 seats, clinching the simple majority required to rule.
Najib's ruling coalition, Barisan Nasional (BN), had 79.
Mahathir told a news conference he expected to be sworn in as prime minister later on Thursday.
"The time for change has come, and I hope the people in power realise this," said Asifa Hanifah, a young woman who joined thousands of opposition supporters in central Kuala Lumpur who waved flags, cheered and honked car horns.
Few had expected Mahathir to prevail against a coalition that has long relied on the support of the country's ethnic-Malay majority.
However, he joined hands with his one-time protege, the jailed politician Anwar Ibrahim, and together their alliance exploited public disenchantment over the cost of living and a multi-billion-dollar scandal that has dogged Najib since 2015.
Mahathir has promised to seek a royal pardon for Anwar if they won the election and, once Anwar is free, to step aside and let him become prime minister.
Several key roads in the heart of the capital, where violence between races has played out in the past, were blocked off by police as evidence grew that Najib's coalition was on the back foot.
The police appealed for calm and said in a statement that, for now, the situation was under control.
Najib's United Malays National Organisation (UMNO) party postponed an evening news conference.
Stocks and the ringgit currency could take a hit on uncertainties over the newly elected alliance's fiscal and economic policies.
Ethnic-Malay Muslims have long tended to support BN for affirmative-action policies that give them government contracts, cheap housing and guaranteed university admissions.
Mahathir's alliance, which counts on urban votes and support from the minority ethnic Chinese and Indian communities, had hoped the veteran Malay leader would win over voters usually loyal to BN. That strategy appeared to have paid off.
Mahathir is a polarising figure and many voters are suspicious of him because of his iron-fist rule as prime minister from 1981 to 2003.
Mahathir was once mentor to the 64-year-old Najib but he left UMNO over the 1Malaysia Development Berhad (1MDB) scandal and joined the opposition.
In an even more unlikely change of heart, Mahathir buried a feud with Anwar, 70, last year and the two agreed to join forces to topple Najib.