CAIRO -- Egypt's military-led interim government laid out an accelerated six-month timetable on Tuesday for a return to civilian democracy, and chose a liberal economist as temporary prime minister, part of an intensified effort to assure Egyptians and the world about its intentions in the aftermath of the mass killing of more than 50 Islamist protesters.
But the Muslim Brotherhood and its Islamist allies rejected the timetable, calling any effort by the interim government illegitimate because of the military's ouster last week of Mohamed Morsi, the Islamist president, who remains under arrest. A top Muslim Brotherhood official, Essam el-Erian, said such a timetable would take Egypt "back to zero."
The Brotherhood also denounced the appointment of a new prime minister, Hazem el-Beblawy, a prominent economic consultant who had supported the 2011 revolution that ousted the old authoritarian government of Hosni Mubarak.
Gehad el-Haddad, a Brotherhood spokesman, said Mr. Beblawy was still the face of Egypt's traditional elite, and that his selection had revealed the interim government's true purpose as an "anti-revolution enshrined by a military coup."
The announcement of the timetable came as Egypt's judicial authorities began to interrogate nearly 650 suspects detained during the deadly mayhem in Cairo on Monday, the worst episode of violence since the revolution more than two years ago that toppled Mr. Mubarak, the predecessor to Mr. Morsi, Egypt's first democratically elected president.
Ahram Online, the Web site of Egypt's leading newspaper, said 60 judicial investigators were looking into the causes of the violence, which left at least 51 pro-Morsi demonstrators dead and hundreds wounded. The police and armed forces have accused Islamist assailants of instigating the fighting, in which they said one police officer was killed and 42 soldiers were wounded. The Muslim Brotherhood and its allies have accused the interim government of committing an unprovoked massacre of unarmed protesters.
In an effort to prop up the interim government, the United Arab Emirates and Saudi Arabia on Tuesday pledged billions of dollars in emergency aid to Egypt, which has suffered a traumatic economic decline and depleted Treasury over the past few years as tourism and foreign investment have collapsed. The governments of Persian Gulf Arab countries, oil-rich kingdoms that strongly opposed Arab Spring political sentiments at home, had longstanding relationships with Mr. Mubarak's old government. The official news agency of the United Arab Emirates, WAM, said it was providing a $1 billion grant and a $2 billion interest-free loan. Reuters reported that the Saudis had approved a packaged totaling $5 billion.
WAM said a delegation visiting with Egypt's interim president, Adli Mansour, had "wished him success in his mission and more progress and prosperity for the Egyptian people in order to pursue its drive towards development, progress and prosperity, enabling the country to play its civilized leading role in Arab and international arenas."
The political transition timetable announced Tuesday would overhaul Egypt's suspended Constitution, elect a new Parliament and choose a president all in the space of about six months.
The release of the new timetable, issued in the name of Mr. Mansour, Adli Mansour, appeared intended to counter doubts about the democratic promises of the generals who removed Mr. Morsi from power. A restoration of a civilian democracy could become a critical determinant of whether the United States continues foreign aid to Egypt.
Under United States law, if Washington officials deem the generals' takeover to be a "coup" or decides that Cairo is moving away from democracy, then the Egyptian military stands to lose about $1.3 billion a year in American aid.
In Washington, the White House spokesman, Jay Carney, said it was "cautiously encouraged by the preliminary plan that was put forward by the interim authorities, and we encourage all parties to participate in dialogue and reconciliation rather than conflict and to resist decisions that would exclude them from the process going forward."
Asked if the billions of dollars in new assistance to Egypt from the United Arab Emirates and Saudi Arabia had diminished the consequence of a possible cut in American aid, Mr. Carney rejected any linkage.
"The issue of our assistance to Egypt encompasses more than the dollar figure involved," he told reporters at a daily briefing. "This is a longstanding partnership, a longstanding commitment to the nation of Egypt and to the Egyptian people by the United States, by both parties -- major parties in the United States."
Previous schedules for Egypt's political transition have often gone unmet, especially under the roughly 18 months of military rule that ended last summer.
Mr. Mansour continued to struggle to assemble an interim cabinet after a handful of candidates for prime minister dropped out or fell away. Before Mr. Mansour chose Mr. Beblawy on Tuesday, he had been preparing to swear in Mohamed ElBaradei, the Noble-prize winning former diplomat and prominent liberal. That plan went awry on Sunday when the one major Islamist party that has supported Mr. Morsi's ouster refused to accept Mr. ElBaradei. Mr. Mansour then decided to make Mr. ElBaradei the interim vice president.
Mr. Mansour, the chief justice of the Supreme Constitutional Court who was tapped by the generals as interim president, has not spoken publicly since he was sworn in. Just before midnight Monday, he issued a "constitutional declaration" laying out a transitional road map that called for the immediate formation of a 10-person committee to revise the charter approved in December. The panel would be composed of six judges chosen by three top courts -- two from each -- along with four Egyptian law professors. It was unclear who would pick the four professors.
The committee is expected to complete its revisions in about a month and then pass them to a larger committee of 50 people representing various government institutions, syndicates and social groups as well as other prominent figures. Some representatives would be selected by their institutions and the others chosen by Mr. Mansour and his cabinet. The military and the police would both pick representatives.
If approved by the larger committee, the revisions would move to a public referendum after about three months, followed in about two more weeks by parliamentary elections. Mr. Mansour's plan called for presidential elections about three months after ratification of the new charter.
Neither Mr. Mansour nor the military commander who named him, Gen. Abdul-Fattah el-Sisi, have said anything about how they plan to include Mr. Morsi's legions of Islamist supporters in the political process.
But on Tuesday, the day before the Muslim holy month of Ramadan here, General Sisi delivered a version of the annual message customarily given by Egypt's civilian president. He urged Muslims to "to live up to the principles" of the holiday in prayer and reflection, assuring all that "the road map is clearly defined and fixed" and "we are marching forward in confident steps in absolute transparency."
Mr. Morsi's party, formed by the Islamists of the Muslim Brotherhood, won nearly half the seats in the last parliamentary election, and more conservative Islamist groups known as Salafis won nearly a quarter more.
Mr. Morsi, who is now in military detention without charges at an undisclosed location, was elected with a little more than 51 percent of the vote more than a year ago. All acknowledged that his popularity has slipped, and millions marched in the streets on the anniversary of Mr. Morsi's inauguration to call for his removal.
But he still has millions of ardent supporters and since his ouster tens of thousands have demonstrated in the streets to call for his reinstatement.
The military-led government has issued arrest warrants for hundreds of top Brotherhood leaders and jailed some of them. It has shut down the Brotherhood's satellite television network and two other Islamist channels.
David D. Kirkpatrick reported from Cairo, and Rick Gladstone from New York.
This article originally appeared in The New York Times.