Earlier this year, residents of the small island of Annobón began noticing withering plants on their farmland and large cracks in their houses.

They attributed the damage to years of dynamite explosions linked to mining operations on the island, a province of Equatorial Guinea that lies in the Gulf of Guinea about 220 miles west of Gabon off the west coast of Africa.

In July, 16 people on the island wrote to the authorities in Malabo, the capital of Equatorial Guinea, expressing concern about the deterioration of the environment and demanding an end to the use of dynamite.

The response came swiftly: within a few days soldiers arrested the letter’s signatories and dozens of activists sympathetic to their cause in raids. Cellular service and internet access were also shut down.

“The troops went from house to house to arrest our relatives,” a relative of one of the detainees said anonymously for fear of being targeted by the authorities. “They took them to police stations on Annobón and then put them in planes without water or food to deport them to Malabo.”

Three months later, only five of the detainees have been released – all elderly women. Those still in custody have been charged with rebellion and “abusive exercise of fundamental rights”.

Eleven captives are being held at Black Beach prison in Malabo, a notorious facility with a reputation for the systematic neglect and brutalisation of inmates. Twenty-six others, including the poet and opposition figure Francisco Ballovera Estrada, are being held at another prison in the eastern town of Mongomo, two sources said, and according to one activist have been denied access to family members and their lawyers.

“They do what they want with your life,” one relative of a detainee said.

Even now, the flow of information from Annobón is still restricted, said Naro Omo-Osagie, the Africa policy and advocacy manager at the New York-based nonprofit Access Now, which was part of a coalition of global digital rights organisations that wrote an open letter in August urging authorities to release the detainees.

“Our sources … managed to get a little information from the island recently from some people who have been able to travel to Malabo via boat in the past few weeks, but they are still unable to reach residents via phone or internet,” she said.

Since unseating the founding president – his uncle – in an August 1979 coup, Teodoro Obiang Nguema Mbasogo has ruled Equatorial Guinea in a draconian manner. Despite having one of the highest GDP per capita rates in Africa, the country’s oil wealth barely trickles down. Obiang’s son, vice-president Teodoro Nguema, once reported to have lost a briefcase containing £250,000, owns an assortment of yachts, while two-thirds of his compatriots live in poverty.

Human rights activists and members of the Equatoguinean diaspora say Annobón’s roughly 5,000 or so inhabitants have endured a particularly grim recent history of human rights abuses and exploitation.

Allocated to Spain during an 18th-century swap of colonies with Portugal, the island was one of a number squeezed into continental Equatorial Guinea after its independence in 1968. The area’s strategic location in the oil-rich Gulf of Guinea makes it important for its parent nation’s territorial claims.

The 6.5 sq mile (17 sq km) territory is the smallest of the country’s eight provinces and its most remote. About 425 miles (685km) from Malabo, Annobón feels more connected to the dual archipelago of São Tomé and Príncipe, 145 miles away. Fa d’Ambô, a Portuguese creole popular in the latter, is also widely spoken among the Annobónese, even though Spanish is the official language in Equatorial Guinea.

The island only has one school, and basic amenities such as electricity and potable drinking water are either lacking or irregular. An airport was built in 2013 but most inhabitants can only afford to leave the island on a weekly ferry or in a berth on a monthly or bi-monthly ship.

For decades, waves of discontent and cries of marginalisation have bubbled under the surface on the island. A cholera epidemic in the 1970s wiped out a third of the population. In the 1980s, it emerged that Obiang had given a UK firm a permit to dump 10m drums of toxic waste there.

In a country where worshipful patriotism is expected – state radio has previously referred to Obiang as a god with “all power over men and things” – a youth-led uprising on Annobón in August 1993 was seen as an affront to the president, and was brutally suppressed.

In July 2022, two of the youths at the centre of the protests three decades earlier unilaterally declared the island’s independence under the aegis of Ambô Legadu, a Spain-based separatist group they had cofounded. One of them, Orlando Cartagena Lagar, was named prime minister of the breakaway republic.

Arbitrary arrests have increased since 2022, and many on the island see the July raids as part of a broader effort to quash dissent. Authorities routinely link activists on the island to Ambô Legadu. “Calling for secession does not constitute a crime itself,” said the Annobonese human rights lawyer Tutu Alicante, who is based in the US. Alicante, who does not support secession himself, said the arrests of activists had violated freedom of expression.

Lagar said recently the Annobónese were facing an unprecedented ecological disaster because of the regime’s drive to extract minerals even near residential areas, which he likened to “a behaviour of extermination”.

The Annobónese had been prevented from making decisions about the island’s development, he said, adding: “The feeling of abandonment is total.”