Mohamed ElBaradei, founder of the liberal Constitution Party, wrote on his Twitter account on Wednesday afternoon: "Vicious attack against peaceful protesters in front of presidential palace without police protection. Regime is leading Egypt into violence and bloodshed."
Clashes erupted at the presidential palace in Cairo's Heliopolis on Wednesday afternoon between supporters of President Mohamed Morsi and anti-Morsi protesters.
According to media statements, there is no security presence at the palace.
On Tuesday, hundreds of thousands marched to the palace to denounce Morsi's November 22 constitutional declaration and what they described as an "unrepresentative constitution." A small number held a sit-in overnight at the palace. Further marches were planned by opposition groups for Wednesday.
The Muslim Brotherhood, meanwhile, has called on supporters to head to the palace on Wednesday to "protect the legitimacy" of the president, following a mass demonstration in support of the president in Giza on Saturday.
Earlier on Wednesday, Egypt's prosecutor-general ordered that investigations be opened regarding a complaint filed against a group of opposition figures – the Constitution Party's Mohamed ElBaradei, Nasserist leader Hamdeen Sabbahi, head of the liberal Al-Wafd party Sayyed Badawi, and former presidential candidate and Mubarak-era foreign minister Amr Moussa.
The complaint was accused the four politicians of forming "a national salvation front" and of mobilising people to pressure the president to withdraw his controversial constitutional declaration.