Staged two days after Islamist rallies exploded into bloodshed, the protests came as a senior official said a new prime minister would be announced today.
Interim president Adly Mansour was leaning towards appointing centre-left lawyer Ziad Bahaa Eldin as prime minister and Nobel Laureate Mohamed ElBaradei as vice president, the president's media advisor Ahmed al-Muslimani told AFP.
If confirmed, Mansour has tried to find a technocrat without the baggage of ElBaradei, whose candidacy outraged Salafi Islamists in a loose coalition that backed president Mohamed Morsi's overthrow by the military on Wednesday.
The Salafis say ElBaradei, viewed as an ardent secularist and top opponent to Islamist Morsi, would have been a divisive premier. But the head of the Salafi Al-Nour party also objected to Bahaa Eldin, because the business lawyer used to belong to ElBaradei's National Salvation Front coalition.
"We don't object to (Eldin) personally, he is an economic pillar," Yunis Makhyun told the Dubai-based Al-Arabiya television channel. "We reject his candidacy because he belonged to the National Salvation Front," he said.
Muslimani has said that while the presidency would listen to objections against prime minister candidates, it could not please everyone with its final choice.
The son of a prominent writer, Bahaa Eldin would be handed the enormous task of bringing a semblance of unity to the new Egypt, just days after the military ousted Morsi.
The development came as protests swelled to an estimated 250,000 in Cairo's Tahrir Square yesterday, epicentre of the 2011 revolution which toppled Hosni Mubarak.
Wave after wave of military aircraft skimmed over the capital, with one formation leaving behind long trails of smoke in black, white and red ; the colours of the Egyptian flag.
"We are on the street to show the world that it was a popular revolution and not a coup that overthrew" Morsi on Wednesday, said a beaming teacher who gave her name as Magda.