Sean Gallup/Getty Images(CAIRO) -- Former Egyptian President Mohammed Morsi was laid to rest in a low-key burial in the early hours of Tuesday, as questions were being raised about his apparently sudden death after suffering a heart attack in a courtroom.

The United Nations High Commissioner for Human Rights called for an independent investigation into the causes of Morsi's Monday death, according to a statement from spokesman Rupert Colville.

"Concerns have been raised regarding the conditions of Mr. Morsi's detention, including access to adequate medical care, as well as sufficient access to his lawyers and family, during his nearly six years in custody. He also appears to have been held in prolonged solitary confinement," Colville said.

Morsi's family and rights groups often complained the diabetic man was not getting proper medical treatment, which Egyptian authorities deny.

"We believe it is clear there must be a thorough independent inquiry into the circumstances of Mr. Morsi's death, including the conditions of his detention," Colville said.

Morsi's quiet burial Tuesday morning stands in stark contrast to how other Egyptian leaders were mourned.

When former President Gamal Abdel-Nasser died, Egypt's streets were packed with mourners and cranked up by loud cries and wails, while former President Anwar El-Sadat's funeral was attended by over 80 world leaders. Their funerals were broadcast over and over again to mark their death anniversaries every year.

Both military strongmen, they were largely hailed as patriotic leaders who served the country at times of war and peace.

But Morsi, Egypt's first freely elected president, would not get a similar treatment.

He was secretly buried in a cemetery accommodating other senior Muslim Brotherhood leaders in Cairo's Nasr City district, with only his family allowed to accompany the corpse to its resting place. A witness told ABC News there was a heavy security presence in a major thoroughfare leading to the cemetery.

Local media hinted that the burial was quiet for security reasons, with one famous pro-state television presenter warning it could lead to clashes between opponents and supporters of Morsi should it be held in public.

"We washed his body in the hospital of Tora Prison and prayed for him at the hospital's mosque," his son, Ahmed, said on his Facebook page.

"He was buried in the graves of the Muslim Brotherhood supreme guides in Nasr City as security authorities rejected [the request] to bury him in Sharqiya," he added, referring to Morsi's hometown in the Nile Delta.

State and privately-owned newspapers briefly mentioned the 67-year-old's death in their morning reports, without making any reference to him as a former president.

Morsi became Egypt's first civilian president when he won a tight election in 2012, a little more than a year after a popular uprising ended the 30-year rule of autocrat Hosni Mubarak.

However, his tenure lasted for just a year until the military ousted him following massive protests against his divisive rule, which alienated even his former revolutionary allies.

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