The English-language Web site of Al Ahram reported Wednesday, citing an unnamed source, that the two Mubarak sons had arrived at the prison unshaven and wearing white training outfits. “Gamal did not look like the Gamal we have seen on TV; he is in a state of total disbelief,” the unnamed source at the prison was quoted as saying.

Word of the detention of the Mubarak brothers ignited exuberant demonstrations in Sharm el Sheik, with a crowd of young men chanting, “15 days!” and “God is great!” in the face of riot police officers who stood guard as the two were driven away, according to amateur video. The Associated Press reported that a crowd pelted the police van with water bottles, stones and flip-flops.

Mubarak critics in Cairo cheered the news as well. “On the road to protecting the revolution,” Mohamed ElBaradei, the former head of the International Atomic Energy Agency who became a critic of Mr. Mubarak and is now a candidate to succeed him, said in a Twitter message. “We now need to focus on achieving its goals.”

Abdullah el-Ashaal, a former Foreign Ministry official who is another presidential candidate, argued that the military council had acceded to the protesters’ demands to prosecute Mr. Mubarak in part to protect the military from public wrath.

“The military wanted to put an end to all the suspicions surrounding it and to the accusation that they were with Mubarak and not with the revolution. Things had reached the point where people started to call for toppling Tantawi,” Mr. Ashaal said. Field Marshal Mohamed Hussein Tantawi, the military’s leader, is now the de facto head of state.

“We want to see Mubarak executed,” Mr. Ashaal added. “Did Mubarak not execute the Egyptian people?”

Others were more cautious. “As gratifying as it is to hear that the unseated dictator has been interrogated and detained, we remain concerned about the lack of a transparent and predictable process for investigating and prosecuting past abuses, whether financial corruption or human rights violations,” Hossam Bahgat, executive director of the Egyptian Initiative for Personal Rights and a leading civil rights advocate, wrote in an e-mail.