The High Commissioner for Human Rights Zeid Ra'ad Al Hussein condemned Sunday's stabbing of Yameen Rasheed and urged a swift and thorough inquiry into the killing, meeting international standards.
"Mr. Rasheed's killing comes in the context of what appears to be an upsurge in arrests and prosecutions of the political opposition," Zeid's spokesperson Ravina Shamdasani said in a statement.
She noted that Rasheed, 29, had received death threats in December and lodged complaints with the police, yet had to push to get his concerns registered.
"We remind the state that it has a duty to ensure that journalists, civil society actors and human rights defenders are able to exercise their rights to freedom of expression and opinion without fear for their safety," Shamdasani said.
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The government of President Abdulla Yameen has already faced censure from the UN rights body over the arrest and jailing of a former president and a former defence minister.
Blogger Ismail Rasheed narrowly escaped death when he was stabbed by an unidentified attacker in 2012.
A journalist with the independent Minivan News, Ahmed Rilwan, was likely abducted in August 2014 and has been missing ever since.
The government has said it will investigate the killing and has called for information leading to the arrest of the perpetrators.
Exiled opposition leader and ex-president Mohamed Nasheed has also demanded an international investigation.
In the wake of Rasheed's death Nasheed, who is living in London, said on Twitter that "a treasured soul has been stolen from us".