The Muslim Brotherhood's Freedom and Justice Party on Saturday rejected Mohamed ElBaradei's appointment as Egypt's interim prime minister, as was announced earlier the same day, according to a senior party official.
"We reject this coup and everything that results from it, including ElBaradei," he told Reuters.
A presidential source on Saturday evening said that ElBaradei, general coordinator of the National Salvation Front opposition coalition, had already been assigned the post of prime minister.
"He accepted and will be sworn in shortly," the source, who spoke on condition of anonymity, told Ahram Online.
ElBaradei was among the liberal leaders who opposed Islamist president Morsi, deposed by the military on Wednesday following nationwide protests.
The overthrow prompted pro-Morsi demonstrations across the country.
ElBaradei has faced growing criticism among Islamists within the past year.
The development could heighten tensions on Sunday, when pro- and anti-Morsi demonstrations – both expected to number in the hundreds of thousands – will be held in Cairo.