On Tuesday night, state media reported Mubarak had suffered a stroke and was on life support. The 84-year-old ousted leader is serving a life sentence for failing to stop the killing of protesters during last year's uprising.
Amid the murkiness over Mubarak's health, Egyptian authorities have delayed the announcement of a winner in the presidential election, which was expected Wednesday.
Tens of thousands flocked to Tahrir Square on Tuesday night to support Morsi, the projected victor of the weekend's election, and express their anger with the ruling military council for delaying the announcement and slowing the promised transition to civilian rule.
The Muslim Brotherhood claimed there was an organized campaign of allegations against it to mar the election and keep Morsi out of the presidency. The accusation raises temperatures and the possibility of a backlash from the Brotherhood if its rival - former prime minister Ahmed Shafiq - is declared the winner.
Egypt's election of a successor to Mubarak was long touted as a landmark moment, the choosing of the country's first civilian president in generations, who was meant to take the reins of power from the generals who have ruled directly since Mubarak's removal on Feb. 11, 2011. Instead, it is shaping into a possible confrontation between the Brotherhood on one side and the military and entrenched elements of Mubarak's old regime.
The Associated Press contributed to this report.