Amnesty International warned Monday of the "seriously" deteriorating health condition of a detained Omani human rights defender on hunger strike since his January 21 arrest in the city of Salalah.
Saeed Jaddad, who led Arab Spring-inspired protests in the sultanate in 2011, was hospitalised on January 23, two days after he started a hunger strike following his arrest.
He was returned to police custody on Monday, and authorities had "ignored" medical advice and were preparing to send him by plane to the capital Muscat for trial, said Amnesty.
"The authorities in Oman are endangering the health and life of activist Saeed Jaddad, who should not be facing trial in the first place," said Amnesty's Philip Luther.
"Rather than putting him at further risk by transferring him from Salalah to Muscat for a court trial, Saeed Jaddad should be released immediately and unconditionally."
Amnesty said that Jaddad, who has a longstanding heart condition, is now refusing liquids and has refused to take medication in protest at his detention.
Jaddad has been charged with "undermining the status and prestige of the state" in his calls for political and social reform in Oman and during a meeting he had with members of the European Parliament in 2013, according to Amnesty.
The prominent activist and blogger, described by Amnesty as a "prisoner of conscience" was arrested the first time between December 10 and 22.
Oman was shaken by uprisings in 2011, prompting Sultan Qaboos to reshuffle his government and expand the consultative assembly.
But scores of activists have been convicted of defaming or using social media networks to insult the sultan, who has ruled for 44 years.
Others have either been convicted or are being tried for taking part in demonstrations calling for political reform.