His arrest was one of the latest in a series which critics say undermines the oil-rich nation's credibility as a host of the United Nations COP29 climate change conference, which starts on Monday.
Mammadli has been locked up since April 29 and risks up to eight years behind bars on smuggling charges human rights groups say are "bogus".
He and activist Bashir Suleymanli had formed a civil society group called Climate of Justice Initiative.
The organisation set out to promote environmental justice in the tightly controlled Caspian nation.
Suleymanli told AFP that the group "was forced to close under government pressure even before it began raising awareness of environmental issues".
"We have no platform through which we could be heard -- not to mention the fact that we will not be able to stage protests during COP29," he said.
International rights groups including Amnesty International and Human Rights Watch, a US senator and the UN special rapporteur on environmental defenders have all denounced the proceedings against Mammadli and demanded his release.
Amnesty said it was part of a "continuing crackdown on civil society activists" ahead of COP29.
The European Parliament has also criticised Azerbaijan's crackdown on critics and said its "ongoing human rights abuses are incompatible" with hosting the summit.
- 'Harsh measures' -
In the streets of Baku, roads are being repaired and buildings getting fresh coats of paint as authorities spruce up the capital in preparation for hosting thousands of foreign guests during COP29, which runs from Monday until November 22.
International rights groups have urged the UN and Council of Europe rights watchdog to "use the momentum of COP29" to "put an end to the persecution of critical voices" in Azerbaijan.
But rather than an easing of repression, Kenan Khalilzade of the Baku-based Ecofront ecological group said the run-up to COP29 has seen more government pressure on activists.
He said he was briefly detained last year during an anti-pollution protest in the remote village of Soyudlu in the country's western Gadabay region.
In 2023, police fired rubber bullets and tear gas at villagers protesting the construction of a pond intended to drain toxic waste from a nearby gold mine.
Locals argued that the pond would cause serious environmental damage to their pastures.
Several villagers were arrested after the violent police crackdown and Soyudlu remained under lockdown for weeks.
"Police threatened me with harsh measures if I ever tried to return to Soyudlu," Khalilzade told AFP.
An investigation by the Organised Crime and Corruption Reporting Project (OCCRP), a global network of investigative journalists, found the mine -- formerly operated by British company Anglo Asian Mining Plc -- is in fact owned by President Ilham Aliyev's two daughters.
- 'Biased and unacceptable' -
Any sign of dissent in Azerbaijan is usually met with a tough response from Aliyev's government, which has faced strong Western criticism for persecuting political opponents and suffocating independent media.
The 62-year-old has ruled the country with an iron fist since 2003, after the death of his father, Azerbaijan's Soviet-era Communist leader and former KGB general Heydar Aliyev.
The Union for Freedom of Political Prisoners of Azerbaijan has published a list of 288 political prisoners, including opposition politicians, rights activists, and journalists.
Among them are several journalists from AbzasMedia and Toplum TV, media outlets critical of Aliyev, and prominent anti-corruption advocate Gubad Ibadoghlu.
In May, Ibadoghlu was allowed to leave house arrest but is under police supervision pending trial. If convicted, he faces up to 17 years in prison.
The fearless fighter for democracy in his authoritarian homeland has argued that Azerbaijan's energy wealth helps Aliyev maintain his family's decades-long grip on power.
In October, Human Rights Watch and the NGO Freedom Now published a report documenting 33 high-profile cases of criminal prosecution, detention and harassment, which they said illustrated the government's efforts to undermine civil society.
Azerbaijan's foreign ministry has rejected the accusations as "biased and unacceptable".
"Conditioning Azerbaijan's presidency of COP29 with inappropriate political motivation contradicts the very essence of the idea of cooperation addressing climate change that Azerbaijan has undertaken," it said in May.
But Khadija Ismayilova, an investigative journalist who has spent months in jail after revealing official corruption, said COP29 delegations should be mindful of Azerbaijan's human rights record.
"Countries that take part in COP29 must be aware that civil society is crushed and downtrodden in Azerbaijan," she said.
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