CAIRO — Protests erupted across Egypt on Friday, as opponents of President Mohammed Morsi clashed with his supporters on a presidential edict that gave him unchecked authority and polarized a divided nation while raising a specter, the president’s critics charged, of a return to autocracy.
In an echo of the uprising 22 months ago, thousands of protesters chanted for the downfall of Morsi’s government in Cairo, while others ransacked the offices of the president’s former party in Suez, Alexandria, and other cities.
Morsi spoke to his supporters in front of the presidential palace here, imploring the public to trust his intentions as he cast himself as a protector of the revolution and a fledgling democracy.
In a speech that was by turns defensive and conciliatory, he ultimately gave no ground to the critics who now were describing him as a pharaoh, in another echo of the insult once reserved for the deposed president, Hosni Mubarak.
“God’s will and elections made me the captain of this ship,’’ Morsi said.
The battles that raged Friday — over power, legitimacy, and the mantle of the revolution — posed a sharp challenge not only to Morsi but also to his opponents, members of secular, leftist, and liberal groups whose crippling divisions have stifled their agenda and left them unable to confront the more popular Islamist movement led by the Muslim Brotherhood.
The crisis over his power grab came just days after the Islamist leader won international praise, including from the United States, for his pragmatism for brokering a truce between Hamas and Israel.
On Friday, the State Department expressed muted concern over Morsi’s decision.
“One of the aspirations of the revolution was to ensure that power would not be overly concentrated in the hands of any one person or institution,’’ said State Department spokeswoman Victoria Nuland.
‘‘The current constitutional vacuum in Egypt can only be resolved by the adoption of a constitution that includes checks and balances, and respects fundamental freedoms, individual rights and the rule of law consistent with Egypt’s international commitments,” she said.
But the White House was notably silent after it had extolled the emerging relationship between President Obama and Morsi earlier this week and credited a series of phone calls between the two men with helping to mediate the cease-fire in Gaza.
For Morsi, who seemed to be saying to the nation that it needed to surrender the last checks on his power in order to save democracy from Mubarak-era judges, the challenge was to convince Egyptians that the ends justified his means.
But even as he tried, thousands of protesters marched to condemn his decision. Clashes broke out between the president’s supporters and his critics, and near Tahrir Square, the riot police fired tear gas and bird shot as protesters hurled stones and set fires.
Since Thursday, when Morsi issued the decree, the president and his supporters have argued that he acted precisely to gain the power to address the complaints of his critics, including the families of protesters killed during the uprising and its aftermath.
By placing his decisions above judicial review, the decree enabled him to replace a public prosecutor who had failed to win convictions against senior officers implicated in the killings of protesters.
The president and his supporters also argued that the decree insulated the Constituent Assembly, which is drafting the constitution, from meddling by Mubarak-era judges.
Since Mubarak’s ouster, the courts have dissolved Parliament, kept a Mubarak loyalist as top prosecutor, and disbanded the first Assembly.
But by ending legal appeals, the decree also removed a safety valve for critics who say the Islamist majority is dominating the drafting of the constitution.
Morsi and the Muslim Brotherhood, the movement he once helped lead, both frustrated with their inability to build consensus, have been accused of conveniently dismissing critics of all types as loyalists of the former government.
In an interview this month, Essam el-Erian, the vice chairman of the Brotherhood’s Freedom and Justice Party, said as much, asserting that the ‘‘the majority’’ of secular or liberal opposition figures were in fact Mubarak loyalists.
Morsi was more forgiving of the opposition Friday, saying he wanted a ‘‘real opposition with awareness’’ while raising the specter of meddling by foreign enemies and a ‘‘few’’ Mubarak loyalists.
Another article in Morsi’s decree, that gives him broad powers to confront unspecified threats — including to ‘‘the revolution’’ — has played into decades-old fears of the Brotherhood as an insular, authoritarian movement shaped by decades as an underground secret society.
But Morsi may have miscalculated the public’s support for the courts, which include judges seen as independent and who have taken stands against both Mubarak and the Muslim Brotherhood.
In Tahrir Square on Friday, derision for the Brotherhood and Morsi’s edicts was widespread. The Brotherhoods’ members were called US agents by one group of chanters, and fundamentalists bent on turning Egypt into Afghanistan by another.
One sign melded Morsi’s face with a youthful picture of Mubarak. “Mohammed Morsi Mubarak,’’ it said.
“They believe only in Islam,’’ said Dr. Ali Abdul Hafiz, a former Brotherhood member and now an opponent of the group. ‘‘Our affiliation is with our country. We want a modern state. How can we believe the Muslim Brotherhood will take it for us?’’
As he spoke, some of Egypt’s leading opposition figures entered the square, including Hamdeen Sabahi, a charismatic Nasserist politician, and Mohamed ElBaradei, a former top UN nuclear official.
Earlier in the day, the two men had joined other prominent figures in a rare show of unity that belied a record of divisions that have rendered the secular-minded opposition largely incoherent.
Morsi’s supporters acknowledge that the administration made a mistake in presenting his decree, without first reaching out to his opponents. Even the president’s advisers were blindsided: One adviser, Samir Morcos, told the state newspaper that he learned of Morsi’s decree on television. He was reported to have quit in protest.