Mansoor, whom Amnesty says is a prisoner of conscience, has been on hunger strike for over three weeks to protest detention conditions and his unfair trial, the London-based group noted.
Mansoor, 49, was sentenced to 10 years in prison last year for what prosecutors said was the dissemination of misinformation in posts on Facebook and Twitter.
Mansoor was accused of using social media to "publish false information and rumours, spread tendentious ideas that would sow sedition, sectarianism and hatred", according to a report published by the UAE's state news agency WAM.
He was also accused of harming "national unity and social peace" and "the state's reputation", WAM said.
During his trial, court proceedings were conducted in almost total secrecy.
Mansoor, one of the most prominent human rights activists in the UAE, received the prestigious Martin Ennals Award for Human Rights Defenders in 2015 for his work in the country.
He was arrested in March 2017 under the UAE's cyber-crime law, triggering an international outcry.
Mansoor was part of a group of activists known as the UAE Five, who were arrested in April 2011.
He was released later that year after a presidential pardon, but authorities confiscated his passport and banned him from leaving the country.
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