UN human rights experts have called on the United Arab Emirates to release the prominent activist Ahmed Mansoor.
Mr Mansoor, 47, was taken away in the early hours of 20 March by security agents, who broke into his home.
His whereabouts are unknown, but state media reported that he was being held on charges of spreading sectarianism and hatred on social media.
The UN experts said they regarded his detention as a "direct attack" on the work of human rights defenders.
"Mr Mansoor's outstanding work in the protection of human rights and the advancement of democracy, as well as his transparent collaboration with UN mechanisms, is of great value not only for the UAE but for the whole region," they emphasised.
The experts urged the authorities to reveal Mr Mansoor's whereabouts, warning that the fact he was being held at an unknown location "puts him at serious risk of ill-treatment and torture".
Last Tuesday, the official WAM news agency quoted the public prosecutor as saying Mr Mansoor was being detained "pending further investigation".
He was accused of using social media "to publish false and misleading information that harm national unity and social harmony and damage the country's reputation".
An electrical engineer and poet, Mr Mansoor has campaigned for freedom of expression, civil and political rights in the UAE since 2006. He has faced repeated intimidation and harassment as a result, according to fellow activists.
In 2011, Mr Mansoor was convicted of "insulting officials" and sentenced to three years in prison, although he was released after eight months.
Two years ago, he was given the Martin Ennals Award for Human Rights Defenders by 10 of the world's leading human rights organisations.