Egypt’s former president Mohamed Morsi has been buried in a remote area of Cairo as his treatment in custody before his death was denounced as torture.

Morsi, the only democratically elected civilian leader in Egypt’s history, fainted in court on Monday and was pronounced dead on arrival in hospital. He was prosecuted on numerous charges after his one-year rule was brought to an end by a military coup in 2013.

His burial in the outlying Nasser City district took place under heavy security. Morsi’s son Ahmed told the Associated Press that Egyptian authorities had refused to allow a burial at the family grounds in Sharqiyah province.

The UN called for an independent investigation into Morsi’s death and his treatment in custody.

Crispin Blunt, the former chair of the foreign affairs select committee in the UK parliament, also called for an investigation.

Blunt led an independent review by British MPs in March last year which concluded that the conditions in which Morsi was being kept were likely to lead to his premature death, and which condemned his treatment as cruel, inhumane and degrading.

Speaking after Morsi’s death, Blunt said: “We found that his detention could meet the threshold for torture in accordance Egyptian and international law. We found that the conditions of Dr Morsi’s detention would be of such continuing interest to the whole chain of command that the current president [the former army chief Abdel Fatah al-Sisi] could in principle be responsible for the crime of torture, which is a crime of universal jurisdiction.”

Blunt said his main concern was that Morsi’s liver disease and diabetes were not being treated. “Dr Morsi’s death in custody is representative of Egypt’s inability to treat prisoners in accordance with both Egyptian and international law,” he said.

Mohamed Morsi during a trial in 2015. Photograph: Anadolu Agency/Getty Images

Morsi was elected president in 2012 after the ousting of the dictator Hosni Mubarak during in the Arab spring. Morsi was a divisive ruler during his year in office, a symbol of Egyptian democracy to some and a conservative authoritarian in the eyes of his opponents, who feared he was putting his Islamist Muslim Brotherhood group before the good of the country.

Military officials arrested Morsi in July 2013, followed by dozens of the Muslim Brotherhood’s top leadership. The former president received a 20-year sentence for the murder of protesters and a life sentence for passing state secrets to Qatar. A death sentence for charges connected to a mass jailbreak in 2011 was overturned in a 2016 retrial.

In 2017 he was sentenced to a further two years in prison for insulting the judiciary. At the time of his death he was being retried on charges of spying for the Palestinian Islamist group Hamas.

News of his death received scant coverage in the Egyptian media, with little mention of his time as president.

International observers and supporters say Morsi’s death was caused by deliberately negligent medical care in prolonged solitary confinement, and that other members of the Muslim Brotherhood’s former leadership risk the same fate. Under the rule of Sisi there have been systematic efforts to crush the group.

Two months of unrest in the summer of 2013 were marked by bursts of extreme violence against Muslim Brotherhood supporters. Human Rights Watch reported that at least 1,150 people were killed in five incidents in which security officials opened fire on demonstrators.

Many of the survivors were arrested and the majority remain in prison. Egypt holds an estimated 60,000 political prisoners, many of them accused of being members of the Brotherhood.

Egypt now considers the Muslim Brotherhood a terrorist organisation and has encouraged western nations such as the US to accept this definition. The head of Egypt’s State Information Service, Diaa Rashwan, was not available for comment when contacted by the Guardian.

Rights groups say political prisoners including accused members of the Muslim Brotherhood are targeted for mistreatment while in mass incarceration. According to the US state department, this includes deliberate prolonged solitary confinement lasting almost six years in some cases, where prisoners are forbidden to leave their cells for more than an hour a day. Authorities have denied accused Brotherhood members and supporters full access to legal assistance, visits from their families and medical treatment.

Hussein Baoumi, of Amnesty International, said members of the Brotherhood were among the political prisoners targeted for particularly intense solitary confinement, a form of torture. He said Egyptian authorities had tightened the noose even further on the Brotherhood in 2015 after the assassination of a former public prosecutor, Hisham Barakat.

Family members of other prominent Muslim Brotherhood figures imprisoned in the maximum-security wing of Cairo’s Tora prison complex fear their relatives could suffer the same fate as Morsi. Relatives and supporters of Dr Essam Haddad, a former representative for international affairs under Morsi, and his son Gehad, a former Muslim Brotherhood spokesman, are concerned that their poor health and a sustained lack of medical treatment could lead to their deaths.

“Both have been held in solitary confinement for six years. My father has suffered four heart attacks and he urgently needs medical attention. My brother Gehad was detained and tortured,” said Abdulla Haddad. “There are many others who are on the verge of death, and unless the international community speak out, many more will die, including my own father and brother.”