Michael O'Flaherty, the pan-European body's top rights official, in the letter to Azerbaijan President Ilham Aliyev urged the immediate release of all such human rights defenders, journalists and civil society activists.

The Azerbaijan capital, Baku, is hosting the COP 29 UN climate change conference despite concerns over human rights under the rule of Aliyev, who took charge after the death of his father Heydar in 2003.

O'Flaherty said activists Akif Gurbanov and Alasgar Mammadli, arrested in March, and Imran Aliyev and Anar Mammadli, arrested in April, had been charged with foreign currency smuggling, tax evasion and forging documents.

Since 2023, more than a dozen staff and journalists of three media outlets -- Abzas Media, Kanal 13 and Toplum TV -- have been arrested on similar charges, he added.

Dated November 4, the letter was only made public now.

"I am concerned at reports that the criminal proceedings... were launched in regard to carrying out their legitimate activities," he said in the letter.

He added: "I ask the relevant authorities in Azerbaijan to immediately release all human rights defenders, journalists and civil society activists who are imprisoned for their legitimate work or for expressing dissenting or critical opinions."

He also expressed concern over reports of "ill-treatment and torture" of activists while in police custody, as well as restrictions on the right of access to a lawyer of their choice.

In its reply to O'Flaherty's letter, also published by the Council of Europe, Azerbaijan said it "rejects all allegations... without any reliable proof" that claim it does not uphold its international obligations.

It said that the activists cited by the commissioner "are accused of smuggling a large amount of funds in foreign currency" and were the target of a criminal case.

Azerbaijan is one of the 46 members of the Council of Europe.

The country has in the past been condemned by the European Court of Human Rights, which is part of the body, for violating the European Convention of Human Rights which the court upholds.