Amateur video posted on the internet appears to show at least one victim of a mob attack in Egypt being dragged through the streets.
Security forces are seen among the crowds in Giza but witnesses said they did not appear to intervene.
The mob killing of four Shi'ite Muslims near Cairo on Sunday has sparked fears of wider sectarian bloodshed in Egypt at a time of grave national crisis.
Egypt's government promised "exemplary punishment" on Monday.
But Shi'ite minority leaders and the liberal opposition accused the government itself, dominated by the Sunni Islamist Muslim Brotherhood, of whipping up sectarian anger over the war in Syria as a means of appeasing its own hardline Salafist allies.
Tensions are running high across Egypt between the government and opponents who plan major rallies this weekend to demand Mursi resign.
There has been scattered political violence and the army, which effectively ran Egypt for decades before the 2011 revolution, warned it would step in to quell any unrest.
Al-Azhar, Cairo's leading Sunni religious establishment, said the killings were contrary to the teachings of Islam and urged "harshest punishment".
Liberal opponents of the Muslim Brotherhood have accused the movement and Mursi of stirring up sectarian passions by joining this month in Sunni calls for jihad against Syria's president and his Shi'ite allies from Lebanon and Iran.