The UN has called for "thorough and transparent investigation" into the death of Mohamed Bites in court, as thousands of people across the Middle East paid their respects to Egypt's former president.
The UN human rights office on Tuesday said the investigation should provide an overview of its treatment during nearly six years of his incarceration.
Egypt's first democratically presided over a Tuesday night after a fatal heart attack in Cairo court, his sons said.
"UN human rights spokesman Rupert Colville said in a statement." Concerns has been raised regarding the conditions of Mr. Morsi's detention,.
"He said." He said. "It should be carried out by a judicial or other competent authority."
Morsi was overthrown on July 3, 2013, after barely in power for a year in a coup staged by current President Abdel Fattah el-Sisi and placed under house arrest before being moved to prison.
Erdogan attends prayer
Morsi's death has led to an outpouring of condolences from around the Middle East.
"I said mourns, for myself and the free people of the world," said Tawakkol Karman, joint recipient of the Nobel Peace Prize.
Al Jazeera Jamal Elshayyal, reporting from Doha said that of hundreds of minorities gathered at a local mosque to pay their respects to the former president.
"The presence of Khaled Meshaal, the former leader of the Palestinian resistance movement, Hamas, is a testament to Morsi is seen as a champion of other causes such as struggle for freedom in Palestine," he said.
Turkish President Recep Tayyip Erdogan attended a prayer service in Istanbul for the former Egyptian president.
At Istanbul's Fatih Mosque, where thousands joined in prayers, Erdogan called Bites of "martyr" and blamed Egypt's "tyrants" for his death, adding that he didn't believe that Bites died of natural causes.
"I don't believe this was normal death," Erdogan, a key supporter of Morsi, said.
The denounced president of the Egyptian authorities for burying Discreetly bites, with only a small number of family members and confidants present.
Exiled Egyptian opposition politician Ayman Nour called Bites "a martyr who was killed deliberately".
Amr Darrag, a senior member of Brotherhood's political party who also lives in exile, said: "Sisi is the murderer and there must be a transparent and independent international investigation."
Rights bodies, Amnesty International and Human Rights Watch, also called for credible investigation into Morsi's death.
HRW said in a statement to Al Jazeera. "The government of Egypt today bears responsibility for his death, given to them with adequate medical care or basic prisoner rights."
"Leah Whitson, executive director of the HRW's Middle East and North Africa division, told Al Jazeera, decreting Morsi's" terrible but entirely predictable "death.
"The Egyptian government has known very clearly about his declining medical state. He had lost a great deal of weight, he had a court in a number of times and he was kept in almost the clock clock."
A leading member of the Muslim Brotherhood group, Morsi won Egypt's first free presidential election in 2012, a year after an uprising that toppled longtime leader Hosni Mubarak.
Turkey's ruling AK Party supported Morsi's government and many Brotherhood members and supporters have fled to Turkey since its activities were banned in Egypt in 2013.
"There are no dissidents and journalists who have been residing in Turkey since the Arab Spring began and people are here to give support for bites' causes," Al Jazeera's Sinem Koseoglu, reporting from Istanbul's Fatih Mosque, said
At the time of his death, Morsi, 67, said he was politically motivated.
Members of now-outlawed Muslim Brotherhood organization, who were arrested in the crackdown following the 2012 coup, are still languishing in jails.
Egypt's government has dismissed accusations that the former president was badly treated.