CAIRO — Egypt’s Parliament on Tuesday selected a 100-member assembly to write a new constitution for the country, heading off a threat from the ruling Supreme Council of the Armed Forces that it would impose its own charter if Parliament failed to act.

The selection of the assembly came just four days before Egyptians go to the polls to select a new president amid heightened concerns about the influence of Islamists in the government. The presidential runoff pits Mohammed Morsi, a member of the Muslim Brotherhood, against Ahmed Shafik, a former air force general who was the last prime minister appointed by toppled President Hosni Mubarak.

Many of Morsi’s opponents have accused the Brotherhood, which dominates Parliament, of trying to control all branches of Egypt’s government, including the writing of the constitution. A previous assembly named to write the charter was dominated by Islamists, who held 70 percent of the seats, and was dissolved after liberal politicians refused to take part.

The new assembly selected Tuesday seemed designed to appeal to a broader spectrum of voters. Among those appointed were representatives of eight political parties, 28 legal and constitutional experts, seven women, 10 Muslim scholars, eight Coptic Christians, seven union representatives and seven members with ties to so-called revolutionary parties or victims of violence during the anti-Mubarak uprising last year.

But Islamists still had a major presence, provoking complaints from 10 political parties that announced they’d boycott the panel’s work.

Among the panel’s members were Emad Abdelghafor, the head of the Nour Party, which draws its support from followers of conservative Salafi Islam; Essam el Erian, the secretary-general of the Muslim Brotherhood’s Freedom and Justice Party; and Sheikh Yasser Burhami, the secretary-general of Egypt’s Salafi Call Movement.

“All this effort was wasted because the political Islam current insists on controlling the assembly and preferring their narrow personal interest to that of the public,” the 10 parties said in a statement, referring to the roles in the assembly of the Freedom and Justice Party and the Nour Party.

Also named to the panel was Gen. Mamdouh Shahin, a member of the Supreme Council of the Armed Forces.

With the 1971 constitution, which had governed during Mubarak’s rule, suspended since last year, Egyptians who will be voting this weekend for a president have no idea what that person’s powers will be. An interim constitution that’s governed the country since March 2011 doesn’t address many key questions, including whether the president has the authority to dissolve Parliament or to declare war. It also doesn’t address who’d become president if the new president died before he’d named a vice president.

The new president is scheduled to take office in July, long before the assembly will have completed its work.

According to the interim constitution, the assembly must draft a new charter in six months, a deadline that legal scholars said was unreasonable. Rushing the process, they warn, will result in an incomplete document that will lead to more instability.

“This is absolute rubbish,” Hossam Issa, a law professor at Ain Shams University in Cairo, said about the timeline.

With the majority of the assembly members belonging to the Muslim Brotherhood, some analysts predicted that a Morsi victory would lead to greater powers for the president. A Shafik victory would lead to few powers, said Fouda Rafat, a law professor at Cairo University.

The biggest mistake of the constitution-writing process already has happened, however, Rafat said.

“They chose to hold the election before writing the constitution. They put, as you Americans say, the cart before horse.”