By KAREEM FAHIM and BEN HUBBARD

Published: June 28, 2013

CAIRO — Thousands of Islamist supporters of President Mohamed Morsi gathered in Cairo on Friday afternoon as Egypt’s highest religious authority warned of the possibility of “civil war” after days of escalating tensions and episodes of deadly violence.

The warning from the religious authority, Al Azhar, the seat of Sunni scholarship in Egypt, came on the eve of mass protests organized by a coalition of Mr. Morsi’s opponents, calling on the president to step down. Mr. Morsi’s supporters have called for counterprotests, leading to fears of violent clashes between the two camps.

The Muslim Brotherhood, Egypt’s largest Islamist party, said that several of its supporters were killed during attacks on its headquarters and on mosques over the last three days. Early Friday, at least one person died in Zagazig, Mr. Morsi’s hometown, according to the Freedom and Justice Party, the Muslim Brotherhood’s political wing.

In comments carried by state news media, a senior scholar at Al-Azhar, Hassan el-Shafei, blamed “ignorant people” for some of the attacks and said that the country needed to be alert “in order for us not to be dragged into a civil war that does not differentiate between supporters and opposition.”

On Friday afternoon, a few thousand Islamists gathered outside the Rabaa al-Adawiya Mosque in Cairo, as the imam urged Muslims to choose peace over violence. Nearby, vendors sold plastic hard hats. Men who bought them said they wanted protection from the blistering sun, but also from rocks, in anticipation of clashes with anti-Morsi protesters.

Some carried long plastic tubes as clubs, one brought a golf club and another a wooden rolling pin — all for self-defense, the men said.

The start of the protests came a day after Mr. Morsi moved aggressively to preserve order and confront his opponents, deploying the army near government ministries and the Suez Canal, starting legal proceedings against several judges and purging critics from a state-appointed body that helps regulate the airwaves.

Though many had hoped that Mr. Morsi would move to defuse calls for mass protests against him this weekend, he instead wielded the might of the state to project power. His message to those challenging his authority was to either work through the political structures that have emerged since the country’s revolution in 2011, or have no say in how the state is run.

“One year is enough!” Mr. Morsi said repeatedly in a televised speech on Wednesday night, threatening to purge holdovers from the clique of former President Hosni Mubarak. He also offered no major concessions to those calling for his ouster, dismissing them as antidemocratic.

“This is the message he is sending, a threat pretty much that his tolerance and patience for so-called subversive acts and extra-constitutional activities will no longer stand,” said Yasser el-Shimy, an analyst with the International Crisis Group. “If you would rather work outside the system, we are going to come after you.”

Mr. Morsi’s decision to shrug off the opposition and forge ahead comes at a time when bitter polarization over the direction of the country threatens to unleash a new wave of unrest.

In his speech, Mr. Morsi went after several individuals he said had broken laws and undermined the state. Action against some of them came swiftly, with government figures appointed by Mr. Morsi moving against them within 24 hours. On Thursday, an accountability body inside the justice ministry opened corruption investigations against a number of judges Mr. Morsi had accused of participating in rigging elections. Gehad El-Haddad, a spokesman for the Muslim Brotherhood, the movement that propelled Mr. Morsi to power, said in an online commentary that 32 judges were under investigation.

The prosecutor on Thursday also slapped a travel ban on Mohammed Amin, the owner of a satellite TV channel that has often been critical of Mr. Morsi since the president accused him of tax evasion. The prosecutor said Mr. Amin was under investigation on allegations he owes more than $60 million in taxes.

“He’s a tax evader — let him pay,” Mr. Morsi said. “He unleashes his channel against us.”

Egypt’s minister of investment on Thursday also purged representatives of three private satellites stations — including Mr. Amin’s — from the board of the Free Media Zone, a state-run body that helps regulate the airwaves.

Mr. Morsi had accused another station owner, Ahmed Bahgat, of owing more than $400,000 to an Egyptian bank.

Mr. Haddad said that Thursday’s moves showed that Mr. Morsi was committed to pursuing his agenda more forcefully.

“What happened today is no more than a fraction of what has been requested from the president since the day of his appointment,” Mr. Haddad said. “These are the basic demands of the revolution, to cleanse the old regime elements from their strongholds in the state.”

Mr. Haddad also said the president had offered no concession to those calling for his ouster because they were working outside the political process, and they most likely would have ignored anything Mr. Morsi offered.

He said he expected Mr. Morsi to work more closely with groups that have proved their popularity at the polls, like the ultraconservative Nour party.

“Most of the Islamist camp thinks the president has been too lenient with the opposition and has failed to deliver on his political promises,” Mr. Haddad said. “And I think he is picking up the pace.”

Planning for the competing protests this weekend continued on Thursday, with the opposition saying its plans had not changed.

“Dr. Morsi’s speech has only increased our insistence on calling for an early presidential election in order to achieve the revolution’s goals, the foremost of which is social justice,” Mohamed ElBaradei, a leader in the opposition’s National Salvation Front, said Thursday at a news conference.

Reflecting fears that the protests could turn violent, the Egyptian Army has been deploying tanks and soldiers near government ministries, at the central bank and at the entrances to some Cairo neighborhoods. It has also enhanced security in cities along the Suez Canal.

Security officials said that weapons had been distributed to the police, and that those being held at local police stations had been transferred to central prisons, apparently to make room for those detained in any unrest.

The American Embassy in Cairo has repeatedly warned American citizens of the potential for violence and will be closed on Sunday, normally the first day of the Egyptian workweek.

Mr. Morsi’s Freedom and Justice Party and the Muslim Brotherhood that founded it took steps to ensure their own security.

The Brotherhood has recently reinforced his Cairo headquarters with thick metal doors, and Mr. Haddad, its spokesman, said the group had hired private security guards to protect its offices.

Protesters have ransacked the group’s offices in the past and sometimes clashed with Brotherhood members. Egypt’s interior minister said recently that the police did not have the resources to protect the offices of all political parties.