Mammadli was detained on April 29, 2024. His arrest was one of a series which critics said undermined Azerbaijan's credibility as a host of the United Nations COP29 climate change conference in November last year.

Mammadli chaired the Election Monitoring and Democracy Studies Centre, which reported irregularities during the February 2024 presidential elections in the tightly controlled, oil-rich nation.

His arrest also came after he participated in events at the UN Human Rights Council, the special rapporteurs said in a statement.

"Defending human rights should never be considered a crime," the experts said.

"There are serious concerns that Mammadli's detention and prosecution may be in retaliation for his human rights work and his engagement with UN mechanisms," they said.

Mammadli risks up to eight years behind bars on smuggling charges, which rights groups claim are bogus.

"Civic participation, independent election monitoring, and cooperation with international mechanisms are the foundation of democratic societies," the experts said.

"Criminalising these activities undermines the rule of law."

The statement was issued by the special rapporteurs on human rights defenders, on freedom of peaceful assembly, the right to freedom of opinion and on health.

They called on Azerbaijan to respect due process, guarantee Mammadli's fundamental rights and provide him with appropriate medical care.

They also urged Baku to end all forms of intimidation against rights activists.

UN special rapporteurs are independent experts appointed by the Human Rights Council to report their findings. They do not speak for the United Nations itself.