A stark new divide appeared to be emerging in Egypt on Friday after the nation’s first democratically elected president asserted nearly unlimited powers, as rival crowds of demonstrators poured into the streets of the capital to express disgust and admiration for the move.

With Islamists lining up behind President Mohamed Morsi and secular leaders rallying against him, the development threatened to wipe away once and for all the unlikely joining of the two forces that brought down Egypt’s longtime leader Hosni Mubarak in 2011.

In a thunderous speech in front of a presidential palace in Cairo, Morsi told thousands of cheering supporters that the sweeping decrees he issued Thursday were intended to defend the revolution that led to Morsi’s election this June.

But just a few miles away in Tahrir Square, thousands more people, most of them well-educated and secular, said that they were resolved to press for another revolution, this time against the Islamist leader who won 52 percent of the presidential vote. Many people said Morsi’s actions were verging on dictatorship.

The Obama administration expressed dismay over Morsi’s action. Victoria Nuland, the State Department spokeswoman, noted that a core aim of the upheaval that toppled Mubarak had been to “ensure that power would not be overly concentrated in the hands of any one person or institution.”

Having congratulated Morsi earlier in the week for his statesmanship in fostering a cease-fire in the Gaza Strip, State Department officials said the United States was seeking an explanation for the new move, which removed all judicial checks over Morsi’s actions.

In his speech, Morsi used harsh language in denouncing judges and prosecutors for doing too little to address the corruption and abuses of the Mubarak years. “There are weevils eating away at Egypt’s nation,” Morsi said.

“It is my duty to move forward with the goals of the revolution and eliminate all of the obstacles which are linked to the past that we hate,” he said.

Clashes broke out across the country in response to the move. The offices of the Muslim Brotherhood and its political wing were torched over Morsi’s close links to those organizations. A Morsi adviser who is a Coptic Christian resigned.

“We have been living in a dictatorship for a very long time, but not like this,” said Yehia el-Gamal, a constitutional law expert who served as a deputy prime minister early in the post-Mubarak era. He said the move went well beyond anything that Mubarak or his predecessors Anwar Sadat and Gamal Abdel Nasser had attempted in the years since 1956 when the monarchy was toppled.

Morsi and his supporters, who include the ultraconservative Nour Party and other groups of political Islamists, have said that the moves were necessary at a time when obstacles erected by judges and prosecutors installed under Mubarak have blocked the new president’s agenda.

His supporters say the action was intended in large part to protect the work of a committee appointed to write a new constitution at a time when Egypt’s highest court had signaled that it might disband that squabbling body. Morsi has said he will relinquish his extraordinary powers after the constitution is written and a new legislature elected.

Egyptians were not alone in questioning Morsi’s actions. In Geneva, a spokesman for the U.N. High Commissioner for Human Rights said that the decree raised serious concerns, while Nuland, the State Department spokeswoman, said the move had “raised concerns for many Egyptians and for the international community.’’

“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,’’ Nuland said.

The crowd at Tahrir Square, while large, had little of the pulsing feel of possibility of the January and February 2011 protests that brought down Mubarak. Those demonstrations were bolstered by the Muslim Brotherhood, whose mobilization was the key factor in ending the leader’s 30-year reign in a matter of weeks. It was the inclusion of many segments of society that gave the opposition its legitimacy.

This time, the Muslim Brotherhood was gathered in front of the ornate Ittihadiya Palace, the seat of an executive branch that once forced them underground. Morsi, one of their own, was delivering his fiery address from a temporary stage that had as its backdrop an enormous banner reading, “The People Support Mohamed Morsi.”

“The people want the implementation of the sharia of God,” the crowded chanted at one point, a reference to laws derived from the Koran and Islamic traditions. “The people want to dissolve the constitutional court.” For the most part, the audience listened attentively to the 45-minute speech.

“There was a plan to destroy the country and the president,” said Walaa Ezzedin, 29, a pharmacist and supporter who came to listen to Morsi. “Those who are objecting are simply against the president. If he had taken the opposite position, they would have objected too.”

The distance between the two sides was evident at the palace and in Tahrir Square, where some of the younger, more affluent Egyptians who took part in the revolution that toppled Mubarak said they were surprised to find themselves aligned with former Mubarak supporters in denouncing Morsi’s moves.

“Overthrowing Morsi is a demand of the revolutionaries and also of the remnants of the old regime,” said Mohamed ElBeshlawy, 32, an accountant. “Now it’s going to be the two groups together. Winds do not blow as the ships wish. There’s going to be another angry revolution.”

Whatever the motivation for Morsi’s move, the effort to shield his government from judicial challenge wold remove “whatever checks and balances exist in Egypt at this point,” said Michele Dunne, a former member of the National Security Council staff under President Obama.

“It’s hard to escape the conclusion that Morsi has overreached, and that he did so partly on the strength of his recent diplomatic victory in Gaza,” said Dunne, now director of the Atlantic Council’s Rafik Hariri Center for the Middle East. She noted that Morsi had previously tried and failed to confront the judiciary in a tussle over the dissolution of Egypt’s parliament.

“He has ended up having to back down, but he keeps trying,’’ Dunne said.

Warrick contributed from Washington. Amer Shakhatreh contributed from Cairo.