Mahathir and Anwar reconciled in recent years in a bid to oust Najib, an extraordinary twist even by the head-spinning standards of Malaysian politics.
However, anxiety has stirred among his supporters as he was not sworn in by the king this morning as expected. He offered his congratulations and said: "We look forward to working with the new Government and leaders, and to continue to strengthen the friendship and cooperation between our two countries".
"There is an urgency here".
Malaysia's government provided a letter of support for 1MDB's 2023 bonds, but hasn't guaranteed the debt, adding to uncertainty amid the leadership change.
Mahathir's victory broke the grip on power of Najib's Barisan Nasional (BN) coalition, which had ruled Malaysia uninterrupted since independence from Britain in 1957. "He followed the events of the new prime minister and how Pakatan Harapan winning has created history and that he is part of it".
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Nielsen issued a statement Thursday night, but it does not directly address the report or that she came close to resigning. Trump has clashed with Nielsen for weeks. "I share his frustration", Nielsen said in a statement.
Mahathir disputed Najib's assertion during a concession speech that Malaysia's king must appoint the new prime minister because no single party has a parliamentary majority, calling it "confusion".
Political analysts have warned that Mr Razak could be trying to buy time to win defections from other parties so he can stay in power.
He was credited with modernizing Malaysia but was also known as a heavy-handed leader who imprisoned opponents and subjugated the courts.
But for the ex-premier, the result could mean more than just the end of his political career - Mahathir has vowed to launch investigations into a headline-grabbing controversy in which Najib is implicated in plundering billions of dollars from sovereign wealth fund 1MDB.
Sri Lankan Prime Minister Ranil Wickremesinghe extending "warmest felicitations" said the "close and cordial relations" between Sri Lanka and Malaysia will be further strengthened. He has denied any wrongdoing.
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Six wounded were brought into the trauma hospital run by Italian medical aid group Emergency, of whom one was dead on arrival. Afghanistan government and security forces have been usually targeted by Taliban and the Islamic State of Iraq.
Earlier, dozens of people - mostly journalists - gathered at an airport near Kuala Lumpur from where Najib and his wife were reported to be leaving for the Indonesian capital, Jakarta, and tried to look into cars entering the complex.
He has pledged to get Mr Ibrahim, who is due out of jail in June, a royal pardon.
There are many strong personalities in the victorious coalition which will have to be reconciled, but none more important than Anwar and Mr Mahathir, who until his defection to the opposition, still described Anwar as morally unfit to lead the country.
But Najib, who faces being investigated by the incoming government for alleged corruption, never appeared and the private jet, owned by Indonesian company Premiair, remained on the tarmac at Subang airport near Kuala Lumpur. The 92-year-old is now the world's oldest leader.
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