Kuwait has released Sheikh Abdullah Salem Al-Sabah, one of two members of the ruling family arrested for tweets deemed critical of the government, he said on Twitter Saturday.
"I asked them [police] to refer me to the public prosecution to defend myself from the horrifying accusations, but they insisted that I sign a pledge and they released me," Sheikh Abdullah said.
Sheikh Abdullah was arrested on Wednesday, while Sheikh Nawaf Malek Al-Sabah was detained a day later. It was not clear if he was also freed.
Sheikh Abdullah said he was questioned by the secret service police on accusations of insulting Emir Sheikh Sabah Al-Ahmad Al-Sabah and instigating against the regime.
He is the grandson of the emir's half-brother, the late Sheikh Abdullah Al-Ahmad Al-Sabah.
The two young royals have written tweets sympathetic to the Kuwaiti opposition, which has been organising protests against an amendment to the electoral law seen as a ploy to produce a pro-government parliament in a snap 1 December general election.
More than 150 protesters and 24 police have been injured in demonstrations since 21 October and the opposition plans another rally on Sunday.
Young royal Sheikh Meshaal Al-Malek Al-Sabah was detained for a few days in July for expressing political views deemed offensive.