Abdullah had chosen Prince Hamzah as his crown prince hours after their father, King Hussein, died of cancer in February 1999. He removed him from the line of succession in 2004.
A senior Jordanian official on Sunday accused the country's former crown prince of conspiring with foreign elements in a "malicious plot" that threatened national security.
Foreign Minister Ayman Safadi told reporters that the plot had been foiled at the “zero hour.”
“Then it was clear they moved from design and planning into action,” Safadi said.
Some 14-16 people are under arrest, in addition to two senior officials close to Prince Hamzah, the official said.
Prince Hamzah, the half-brother of Abdullah II, said Saturday he has been placed under house arrest and accused the country's "ruling system" of incompetence and corruption.
Prince Hamzah's videotaped statement came after the country's official news agency reported that two former senior officials and other suspects had been arrested for “security reasons."
Safadi did not identify the foreign countries allegedly involved in the plot. But he said a longtime senior official who has business ties in several Gulf Arab states, Bassem Ibrahim Awadallah, was involved and had been planning on leaving the country.
Asked whether Hamzah could face charges, Safadi said that for the time being there were “amicable” attempts to deal with him, but added that “the kingdom’s stability and security transcends” everything.
Jordan's allies stand by Abdullah
The unprecedented incident has raised concerns about stability in a country seen as a key Western ally in a volatile region.
But the crisis appeared to have little immediate effect on outside support for Abdullah.
“King Abdullah is a key partner of the United States, and he has our full support,” US State Department spokesman Ned Price said.
The US considers Jordan a major ally, granting it access to military equipment and assistance. The kingdom hosts some 3,000 American troops.
Gulf Arab countries also immediately issued statements backing the king and his government.
The state-run Saudi Press Agency said the kingdom’s royal court supported King Abdullah’s efforts “to maintain security and stability and defuse every attempt to influence them.”
Israeli Defense Minister Benny Gantz called Jordan a "strategic ally" and dismissed the turmoil as an “internal Jordanian matter.”
Queen Noor defends son
Early on Sunday, Hamzah's mother, Queen Noor, expressed sympathy for "innocent victims," claiming she was also placed under house arrest.
"Praying that truth and justice will prevail for all the innocent victims of this wicked slander. God bless and keep them safe,'' she tweeted.
In a video leaked to the BBC, Hamzah — a former crown prince stripped of his title in 2004 — said he was visited early Saturday by the country's military chief and told he was not allowed to go out, communicate with people or meet with them.
He said his security detail was removed, and his phone and Internet service had been cut. He said he was speaking over satellite Internet and expected that service to be cut as well. The BBC said it received the statement from Hamzah's lawyer.
Hamzah said he had been informed he was being punished for taking part in meetings in which the king had been criticized, though he said he was not accused of joining in the criticism.
He then lashed out at the "ruling system" without mentioning the king by name, saying it had decided “that its personal interests, that its financial interests, that its corruption is more important than the lives and dignity and futures of the 10 million people that live here.”
“I’m not part of any conspiracy or nefarious organization or foreign-backed group, as is always the claim here for anyone who speaks out," he said. "There are members of this family who still love this country, who care for (its people) and will put them above all else.”
“Apparently, that is a crime worthy of isolation, threats and now being cut off,” he added.
It is rare for a senior member of the ruling family to express such harsh criticism of the government, and any sign of instability in Jordan is likely to raise concerns among the country's Western allies.
Hamzah is a popular figure in Jordan. He is seen as religious and modest, in touch with the common people and similar to his beloved father, the late King Hussein. He has criticized the government in the past, accusing officials of “failed management” after they approved an income tax law in 2018.
The country's top general had earlier denied that Hamzah was detained or under house arrest. Hamzah was asked to “stop some movements and activities that are being used to target Jordan's security and stability,” Gen. Yousef Huneiti was quoted as saying by the official Petra news agency.
He said an investigation was ongoing and its results would be made public “in a transparent and clear form.”
“No one is above the law, and Jordan’s security and stability are above all,” he added.
Petra had earlier reported that Sharif Hassan bin Zaid, a member of the royal family, and Bassem Ibrahim Awadallah, a former head of the royal court, were detained. Awadallah also previously served as planning minister and finance minister and has private business interests throughout the Gulf region.
The agency did not provide further details or name the others who were arrested.
Abdullah has ruled Jordan since the 1999 death of his father, King Hussein, who ruled the country for close to a half-century. Abdullah has cultivated close relations with U.S. and other Western leaders over the years, and Jordan was a key ally in the war against the Islamic State group. The country borders Israel, the occupied West Bank, Syria, Iraq, and Saudi Arabia.
“We are closely following the reports and in touch with Jordanian officials,” State Department spokesman Ned Price said. “King Abdullah is a key partner of the United States, and he has our full support.”
Saudi Arabia's official news agency said the kingdom “confirmed its full support to Jordan and its king and crown prince in all decisions and procedures to maintain security and stability and defuse any attempt to affect them.”
Jordan’s economy has been battered by the coronavirus pandemic. The country, with a population of around 10 million, also hosts more than 600,000 Syrian refugees.
Jordan made peace with Israel in 1994. The countries maintain close security ties, but relations have otherwise been tense in recent years, largely due to differences linked to Israel’s conflict with the Palestinians. Jordan is home to more than 2 million Palestinian refugees, most of whom have Jordanian citizenship. Israel's Foreign Ministry declined comment.
Stability in Jordan and the status of the king has long been a matter of concern, particularly during the Trump administration, which gave unprecedented support to Israel and sought to isolate the Palestinians, including by slashing funding for Palestinian refugees.
In early 2018, as then-President Donald Trump was threatening to cut aid to countries that did not support U.S. policies, the administration boosted assistance to Jordan by more than $1 billion over five years.
Abdullah stripped his half-brother Hamzah of his title as crown prince in 2004, saying he had decided to “free” him from the "constraints of the position" in order to allow him to take on other responsibilities. The move was seen at the time as part of Abdullah's consolidation of power five years after the succession.
The current crown prince is Abdullah’s oldest son, Hussein, who is 26.
Jordan's ruling family traces its lineage back to Islam's Prophet Muhammad. Abdullah had chosen Hamzah as his crown prince hours after their father died of cancer in February 1999. The designation was out of respect for King Hussein, who was known to have favored Hamzah the most among his 11 children from four marriages.
Until now, Abdullah and Hamzah have not displayed any open rivalry. In the videotaped statement, a portrait of King Hussein could be seen on the wall behind the prince.