* Ban came minutes after Najib announced he would vacation abroad
* Dozens gathered at airport amid reports he was planning to leave
* New prime minister Mahathir to reopen probe into 1MDB scandal
* US Attorney-General has called case "kleptocracy at its worst" (Writes through after order banning Najib from leaving country)
KUALA LUMPUR, May 12 (Reuters) - Malaysian authorities barred ousted Prime Minister Najib Razak and his wife from leaving the country on Saturday, amid reports that the government was reopening investigations into a multi-billion-dollar graft scandal at a state fund he founded.
The order by immigration authorities came minutes after Najib said in a Facebook post that he and his wife, Rosmah Mansor, were taking a week-long holiday overseas to rest after his thumping defeat in Wednesday's general election.
"The Malaysian Immigration Department would like to confirm that Najib Razak and Rosmah Mansor have just been blacklisted from leaving the country," the agency said on its official Facebook page. It gave no reason.
Moments later again, Najib said in a Twitter message that he would respect the decision and would remain in the country. He was last known to be at his home in the upmarket Bukit Tunku neighborhood of Kuala Lumpur.
Two sources told Reuters on Friday that new Prime Minister Mahathir Mohamad planned to reopen investigations into the graft scandal at state fund 1Malaysia Development Berhad (1MDB) that has plagued Najib since 2015.
Mahathir would appoint a finance ministry adviser to "restart the 1MDB probe and bring back the money," said one source, who worked closely with his campaign team. The second source, a lawmaker, said the announcement could be made on Saturday when Mahathir names members of his new cabinet.
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.
"I'm here to catch the thief," said a man in the crowd, as riot police stood on guard. There was no sign that Najib or Rosmah had come to the airport.
Reports widely circulated on social media and local media said the couple were named on the flight manifest of a private jet scheduled to depart for Jakarta at 10:00 a.m. (0200 GMT).
But police later said there was no flight due to leave the Sultan Abdul Aziz Shah Airport with Najib.
"NOT SEEKING REVENGE"
Najib, 64, lost this week's general election to an alliance headed by Mahathir, his mentor-turned-rival, at least partly because of popular disgust over the 1MDB scandal.
Mahathir, who was the country's premier for 22 years and was sworn in again as prime minister on Thursday, said he would not be looking for a scapegoat in the scandal.
"We are not seeking revenge," he said. "What we want is to restore the rule of law ... if the law says that Najib has done something wrong, then he will have to face the consequences."
News broke in 2015 that about $700 million allegedly stolen from 1MDB had made its way into Najib's personal bank accounts.
He denied any wrongdoing and was cleared by Malaysia's attorney-general, even as U.S. authorities alleged that over $4.5 billion was stolen from the fund in a fraud orchestrated by a financier known to be close to Najib and his family.
U.S. Attorney-General Jeff Sessions called the 1MDB scandal "kleptocracy at its worst" and the fund is the subject of money-laundering investigations in at least six countries, including Switzerland, Singapore and the United States.
Filings by the U.S. Justice Department in a civil lawsuit indicated nearly $30 million of the money stolen was used to buy jewelry for Rosmah, including a rare 22-carat pink diamond set in a necklace.
In a Twitter message on Friday, Najib said he was praying that, "after this divisive period," Malaysia would unite.
"I apologize for any shortcomings and mistakes, and I thank you, the people, for the opportunity to lead our great nation."
(Writing by Raju Gopalakrishnan Additional reporting by A.Ananthalakshmi, Praveen Menon, Emily Chow, Liz Lee and Joseph Sipalan; Editing by John Chalmers)