Supporters of former Egyptian President Mohamed Morsi have accused security forces of "murdering" him, The Independent newspaper reported.
Morsi, Egypt's first freely-elected president, died after falling into a coma inside his soundproof glass cage during his trial on "espionage" charges.
Abdullah al-Haddad, whose father and brother were standing trial alongside Morsi, said security forces left the former president "slumped on the floor" of his cage for more than 20 minutes.
"He was left slumped for while till the guards took him out," al-Haddad told The Independent.
He said security forces failed to take the necessary measures to provide first aid to Morsi inside his cage.
"An ambulance arrived after 30 minutes," he said. "Other detainees were first to notice his collapse, they started shouting. Some of them, who are doctors, asked the guards to let them treat him or give him first aid."
Egypt's public prosecutor earlier said the 67-year-old leader was immediately rushed to the hospital, where he was pronounced dead.
On Tuesday, Turkish President Recep Tayyip Erdoğan attended a funeral prayer in absentia for Morsi where he expressed suspicion about his death.
"Whether it was a normal death, or there were some other elements involved, this was suspicious," Erdoğan said after the funeral prayer at Fatih Mosque in Istanbul. "Personally, I do not believe that it was a normal death."
The UN as well called Tuesday for launching independent investigations into Morsi's demise.
In response, Cairo blasted the UN's statement, saying it was an attempt to "politicize" what it called "natural demise" of Morsi.
Egypt's Foreign Ministry said "the statement issued by the spokesman of High Commissioner for Human Rights will be addressed at the highest level," citing the lack of "objectivity" and "integrity."
Morsi, a leading member of Egypt's Muslim Brotherhood, won Egypt's first free presidential election in 2012.
After only a year in office, however, he was ousted and imprisoned in a military coup led by then-Defense Minister and current President Abdel-Fattah al-Sisi.
At the time of his death, Morsi faced a host of legal charges, which he, along with numerous human rights groups and independent observers, said were politically motivated.
