Violent clashes between Islamists and security forces broke out in front of Cairo University following a televised address by Egypt's president.
At least 16 people were reported dead with more than 200 others injured.
There were also earlier confrontations between pro and anti President Mursi supporters in which at least seven people died.
Mohamed Mursi's message to respect the legitimacy of his election as president was ridiculed by opponents in Tahrir Square:
"Our legitimacy began in Tahrir Square when the first martyr fell, we kept silent without threatening revenge. We will find a solution for our rights to be restored, " said one anti-Mursi protester.
Troops are on alert amid warnings of widespread violence once an army ultimatum to the president to share power with his opponents passes Wednesday evening. In a message on a social website the military has already said it is willing to shed its own blood to defend Egypt's people.
Source: Euronews
