Jordan said Sunday it had stopped a plot to "destabilise" the kingdom involving Prince Hamzah bin Hussein, a half-brother of King Abdullah, and arrested multiple suspects.
Hamzah -- a former crown prince who lost that title in 2004 -- and the others had worked with a foreign power to "undermine the security" of Jordan, Deputy Prime Minister Ayman Safadi said.
Prince Hamzah, 41, had Saturday released a video message via the BBC in which he accused Jordan's rulers of nepotism and corruption and charged that he had been placed under house arrest.
Hamzah in his message, which he said he was sending via satellite phone, lashed out at Jordan's "ruling system" and said several of his friends had been arrested, his security detail removed and his internet and phone lines cut.
He denied being part of "any conspiracy or nefarious organisation", but said the country had "become stymied in corruption, in nepotism and in misrule" and that nobody was allowed to criticise the authorities.
The official newspaper Al-Rai warned Sunday that Jordan's "security and stability" were a "red line that must not be crossed or even approached".
Washington and Gulf allies stressed their support for the pro-Western government in Amman, seen as an anchor of stability in the Middle East.
Official news agency Petra said among those arrested for "security reasons" were former close aides to the royal family Bassem Awadallah, chief of the royal court in 2007-2008, and Sherif Hassan bin Zaid.
Safadi said Sunday an additional 14 to 16 suspects had been arrested.
Hamzah's mother, Queen Noor, tweeted that she was "praying that truth and justice will prevail for all the innocent victims of this wicked slander. God bless and keep them safe."
- US closely watching -
Hamzah is the eldest son of the late King Hussein and American-born Queen Noor.
Abdullah had appointed Hamzah, a popular figure close to tribal leaders, crown prince in 1999 in line with Hussein's dying wishes, but in 2004 stripped him of the title and gave it to his own eldest son Hussein.
A Jordanian analyst who did not want to be named for security reasons said Hamzah had recently "stepped up his criticism of what he described as corruption within the government in front of his circle of friends".
According to the same source, "there is certainly resentment on his part, because he has never digested losing his title of crown prince".
The army denied Saturday that Hamzah, who holds no official position, had been detained.
"What has been published about the arrest of Prince Hamzah is not true," said Joint Chiefs of Staff head Major General Yousef Huneiti.
But the prince had been "asked to stop some activities that could be used to shake the stability and security of Jordan", he said.
The Washington Post said the former crown prince was "placed under restriction" as part of a probe into an alleged plot to unseat the king.
"The move followed the discovery of what palace officials described as a complex and far-reaching plot," it said, quoting a senior Middle East intelligence official.
The alleged scheme "included at least one other Jordanian royal as well as tribal leaders and members of the country's security establishment," the Post added.
- Upcoming centenary -
US State Department spokesman Ned Price said Washington was "closely following" the events in its close regional ally.
"King Abdullah is a key partner of the United States, and he has our full support," he said.
The United Arab Emirates and Saudi Arabia were also quick to voice their full support for Jordan's king and all steps taken to ensure stability, as were the Arab League and the Gulf Cooperation Council.
Israel's Defense Minister Benny Gantz said "Jordan is a neighbour and strategic ally with whom we have peaceful relations. We need to do everything necessary to maintain that alliance, which is ongoing for over 30 years".
Awadallah, a former finance and planning minister educated in the United States, was close to the king but has also been a controversial figure in Jordan.
Before becoming royal court chief in 2007, he was head of the king's cabinet.
He had played a key role pushing for economic reforms in the country before he resigned in 2008 amid criticism over alleged interference in sensitive political and economic issues.
Saturday's security sweep came as Jordan prepares to mark 100 years since the new kingdom was established alongside Palestine under British mandate, then named Transjordan.
It declared independence in 1946. Despite having little oil wealth and severely lacking water, Jordan has managed to survive repeated wars in the region which have sent waves of refugees across its borders.