Saudi's King Salman approved the arrests of three senior royals over the weekend, local sources told Reuters.
On Friday, his son, Mohammed Bin Salman, detained the king's younger brother, and Prince Mohammed Bin Nayef, the king's nephew and his half-brother Nawaf.
Three of the sources, including a regional source, said Mohammed and Nawaf Bin Nayef were picked up at a private desert camp on Friday. Two sources said Ahmed was taken from his home.
Crown Prince Mohammed "accused them of conducting contacts with foreign powers, including the Americans and others, to carry out a coup d'etat," the regional source said.
"With these arrests, MbS consolidated his full grip on power. It's over with this purge," the source added, indicating that no rivals remain to challenge his succession to the throne.
Another source said the princes were accused of "treason". A third source said they had been discussing a coup with the support of powerful tribes but had not reached advanced stages.
The Saudi government media office did not respond to a request for comment on the detentions, first reported by the Wall Street Journal.
- Prince Ahmed Bin Abdulaziz, King Salman's brother
- Prince Mohammed Bin Nayef, king's nephew
- Prince Nawaf Bin Nayef, king's nephew
It was unclear where the princes are being held. There was no way to contact them for comment on the coup allegations.
"They [princes] have to be treated with dignity," the third source said, referring to their stature within the family.
The regional source and another source said King Salman had approved the move and described him as mentally and physically sound.
The 84-year-old monarch met British Foreign Secretary Dominic Raab on Thursday in Riyadh. King Salman and the crown prince attended a cabinet meeting on Tuesday.