CAIRO -- A key voice in the Egyptian opposition movement threatened fresh nationwide protests and a campaign of civil disobedience Monday if embattled President Mohamed Morsi refuses to step down to make way for early elections.

The announcement by the youth movement, known as Rebel, came as Morsi and his ruling Muslim Brotherhood party remained in a political standoff with opposition forces after rival rallies left 16 people dead and nearly 800 injured in clashes between Islamists and anti-Morsi protesters.

Cairo and other cities were largely quiet early Monday but tensions were high as the nation’s political divisions deepened.

PHOTOS: Massive protest against President Morsi in Egypt

The massive protests against Morsi on Sunday, the largest since the demonstrations that brought down Hosni Mubarak two years ago, were a stark repudiation of the Brotherhood’s Islamist agenda and authoritarian tendencies. The president finds himself between the military, which will not tolerate sustained unrest, and an opposition movement similar in passion to the one that drove out Mubarak.

“There is no way to accept any half-measures,” said the announcement by Rebel, which claims to have collected 22 million signatures on petitions to unseat Morsi. "There is no alternative other than the peaceful end of power of the Muslim Brotherhood and its representative, Mohamed Morsi."

The group said it will instigate “complete civil disobedience” if Morsi doesn’t resign by 5 p.m. Tuesday. Rebel called on “state institutions, including the army, police and judiciary, to clearly side with the popular will.”

Despite hundreds of thousands of protesters – some estimates suggested millions – arrayed against him, Morsi is not likely to relinquish power. He is Egypt’s first freely elected president and has support from nearly all Islamist factions, including moderates and ultraconservative Salafis.

The Brotherhood, once an outlawed organization, has waited more than 80 years to rise to political power. Its Cairo headquarters was attacked and torched Sunday night and at least 10 of its members have been killed in recent days. But the group has remained defiant, accusing the opposition of attempting to bypass the ballot box with anarchy.

“There was no civil war as the liars advertised,” Essam Erian, a prominent Brotherhood official, wrote on his Facebook page, referring to fears of widespread factional fighting. “And there will be no military coup as the losers want.”

But Morsi and the Brotherhood have yet to fix Egypt’s many ills, including poverty, power outages, plummeting foreign reserves, rising crime and nascent sectarianism.

“The Brotherhood has promised it would do things but nothing has been done,” said Abdelrahman Yousry, a high school student standing in front of the charred Brotherhood headquarters. “There’s no security, no clean streets, no jobs. Morsi has failed at everything. He increased the crises and has used religion to divide the Egyptian people.”

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jeffrey.fleishman@latimes.com

Special correspondent Ingy Hassieb contributed to this report