CAIRO -- The public prosecutor on Monday ordered the arrest of five anti-Islamist political activists on charges of using social media to incite violence against the Muslim Brotherhood. The order stirred accusations of a vendetta by the group's close ally, President Mohamed Morsi.
Egyptians are already on guard against the possibility that their first freely elected president may seek to become a new autocrat, and some said they feared that the arrest warrants were the first clear example that Mr. Morsi's government was using law enforcement as a political tool to punish his critics.
The arrests arose out of an attack by anti-Islamist activists on the Muslim Brotherhood's headquarters in Cairo on Friday night. As many as a thousand of the group's opponents arrived armed with sticks, knives and at least a few guns, and they seemed intent on burning down the headquarters.
A roughly equal number of Brotherhood supporters surrounded the building to defend it, many bused in for the night, and for a time the two sides clashed in the streets. Then an overwhelming force of riot police officers separated the two sides, using tear gas to drive back the attackers. By the end of the night, several Brotherhood buses had been burned. Health officials reported more than 100 injuries, although it was impossible to confirm how many were on each side.
Afterward, Mr. Morsi sought to blame his political opponents for the attack and vowed action against those who had incited the violence. In a message on Twitter on Sunday, he castigated opposition leaders, accusing them of "providing a political cover for violence."
"Whoever is found to be involved in promoting violence through the media will not escape punishment," Mr. Morsi said in a short speech later on Sunday. He also said he was prepared "to impose exceptional measures to restore domestic order."
Mr. Morsi's political opponents have already denounced him since last fall for picking his own public prosecutor, Talaat Ibrahim, by using a presidential decree to circumvent Egyptian law, under which a president cannot normally replace a public prosecutor. The appointment immediately raised questions about the potential political use of the post.
On Monday, Mr. Morsi's critics said Mr. Ibrahim appeared to be following through on precisely the threat Mr. Morsi made just a day earlier. In a statement, the public prosecutor said the five defendants singled out for arrest had used Facebook and Twitter to urge others to "burn down the headquarters of the Muslim Brotherhood and to murder some of its members."
Before the disturbance, one defendant, Alaa Abdel Fattah, argued against violence, but he suggested in an online commentary on Saturday that what he considered to be Mr. Morsi's authoritarianism might make a violent response legitimate. But Mr. Abdel Fattah, a prominent activist previously imprisoned for months for challenging military rule, was writing as part of an abstract, intellectual discussion after the fact.
The others named in the arrest warrant are the activists Ahmed Douma, Kareem al-Shaer, Hazem Abdel Azeem and Ahmed al-Sahafy.
On Tuesday, a supporter of Mr. Morsi provided a bellicose Twitter post from one of the accused activists to substantiate the charge of incitement. Around the time of the attack on the Muslim Brotherhood headquarters on Friday, Mr. Abdel Azeem said in a post that he had seen an Islamist message suggesting an "auction" for the back of his neck.
He responded by describing the Brotherhood's members as "sheep" and suggesting that he would pay "an old shoe" in an auction for Mr. Morsi's scalp.
Opponents of the Brotherhood often mock its members as "sheep" for their supposed obedience to the group, and some of the responses to Mr. Abdel Azeem's post sound like violent threats: "We will skin then slaughter," read one.
But that message was a reference to a catchphrase from a well-known Egyptian comedy about an inept butcher, and other responses suggest that Mr. Abdel Azeem's comments were taken as a black joke. Mr. Abdel Azeem, who was injured during Friday's clashes, also sent Twitter messages urging the protesters to remain peaceful.
In television interviews on Monday night, several defendants accused the public prosecutor of selectively targeting Mr. Morsi's critics while ignoring testimony, videos and other evidence that Islamists had also used violence on their opponents that night, as well as in street clashes over the last four months. One video posted on the Internet, in fact, captured footage of one of Mr. Morsi's Islamist supporters beating Mr. Douma outside the headquarters earlier last week.
The prosecutor is either "literally blind" or "complicit," Mr. Douma said in a television interview, contending that the prosecutor was "only following orders."
In response, Mr. Morsi's supporters pointed out that earlier in the week, the prosecutor ordered the arrest of three bodyguards of a Brotherhood leader, Khairat el-Shater, on charges of using violence against the opposition protesters.
Pakinam el-Sharkawy, a political adviser to Mr. Morsi, complained that the president's critics had applied a "double standard." The president did not prejudge this case and expected a fair trial, she said, but at the same time the law must distinguish between political expression and criminal violence. "Here," Ms. Sharkawy said, "the law must have a just sword capable of protecting rights, freedoms and social peace."
This article originally appeared in The New York Times.