Egyptians are to go to the polls for a second and final day of voting to elect their first president since Hosni Mubarak was forced from office in 2011.
The vote also comes amid a bitter row over the dissolution of parliament following a court ruling on Thursday.
Mr Mursi's Muslim Brotherhood has denounced the step as unlawful and a coup against democracy.
The movement urged Egyptians to protect their revolution after the ruling Supreme Council of Armed Forces (Scaf) declared the parliament null and void on Saturday.
Two days earlier, the Supreme Constitutional Court ruled that last year's legislative polls were unconstitutional, in a decision made by judges appointed under Mr Mubarak.
The dispute has laid bare the fears of some that the military council is trying to consolidate power and resist the democratic changes demanded during last year's demonstrations.
Mr Shafiq, Mubarak's last prime minister, has campaigned on a platform of a return to stability and law-and-order which, correspondents say, many find attractive after months of political turmoil.
But to his critics, the former air force officer is the army's unofficial candidate and a symbol of the autocratic days under Mubarak.
Mr Mursi, meanwhile, has cast himself as a revolutionary and part of the movement that overthrew Mubarak, and has promised economic and political reform.
He has also softened his religious stance in an attempt to attract liberals and minorities.
His Freedom and Justice Party won almost half of seats in the legislature in the 2011 polls.
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