Prince Hamza bin Hussein was formerly Crown Prince of Jordan.
Videos posted online showed a heavy police deployment in the Dabouq area near the royal palaces on the edge of Amman, while former crown prince Hamzah bin Hussein said he was confined to his home.
A security investigation has been launched and a former minister, a junior member of the royal family and unnamed others have been detained.
Prince Hamzah said in a video recording he was under house arrest and had been told to stay at home and not contact anyone.
"No one is above the law and Jordan's security and stability are above all", he told the official Petra news agency.
The state news agency said on Saturday that Bassem Awadallah, a US -educated long-time confidant of the king who later became minister of finance and also advisor to Saudi Crown Prince Mohammed bin Salman, and royal family member Sharif Hassan Ben Zaid had been detained, along with others.
The pair were detained for "security reasons", Petra said, quoting a security source.
The widow of Jordan's late king defended her son, Prince Hamzah, against allegations by the authorities that he carried out actions targeting "security and stability" in the kingdom.
"Praying that truth and justice will prevail for all the innocent victims of this wicked slander. God bless and keep them safe", she said on Twitter.
Many countries and groups have reacted saying King Abdullah has their full support.
Sherif is a title given to those close to the royal family in Jordan.
The United States, which is allied with Jordan in its campaign against the Islamic State (IS) group, described the monarch as a key partner who has its full support.
The half-brother of Jordan's head of state King Abdullah II is believed to have been placed under house arrest amid reports of an alleged coup attempt. "The official media is totally silent and we know that there is much more to the story that we are not seeing".
Regional powers including Egypt, Turkey, and Saudi Arabia have voiced support for King Abdullah in the wake of the operation.
He emphasised that the safety and stability of Jordan are a top priority, adding that needed measures will be taken to safeguard the country.
He denied conspiracy, but accused Jordan's leaders of corruption and incompetence.
Some commentators said the prince's criticism of corruption in the kingdom struck a chord with many in the country.
Saudi Arabia said it supports King Abdullah's decision to maintain the security of his country, according to a statement published by the Saudi Press Agency Saturday.
He said some 14-16 people are under arrest, in addition to two senior officials close to Prince Hamzah.
The unprecedented incident has raised concerns about stability in a country seen as a key Western ally in a volatile region and drawn an outpouring of support for Abdullah.
Whatever damage the crisis might have inside Jordan, however, appeared to have little immediate effect on outside support for Abdullah. Since then, King Abdullah has been a top US ally, often allowing USA troops to stage operations from Jordanian territory and participating in the anti-Islamic State campaign. The kingdom, which has scant resources, received some $1.5 billion in assistance from the U.S.in 2020 - a result of Abdullah's popularity among congressional leaders.
As the eldest son of Queen Nour (nee Lisa Halaby), his father's fourth and final queen, the now-41-year-old prince was thought to be King Hussein's top choice as successor; he was passed over because he was still in school. Awadallah also previously served as planning minister and finance minister.
Late Saturday night, Jordan's Chief of Staff Maj.
It declared independence in 1946.