Egyptian security forces arrested 15 armed bodyguards of the number two in the ruling Muslim Brotherhood, Khairat El-Shater, on Monday after an exchange of fire in which no one was injured, security sources said.
The sources said the shootout occurred when security forces went to arrest the guards for alleged unlawful possession of firearms they are suspected of having used to shoot at protesters attacking the Brotherhood's headquarters on Sunday.
Eight people were killed and more than a dozen injured in clashes around the Brotherhood building on a suburban hilltop overlooking Cairo.
Shater's family telephoned Al Jazeera television station to report that his home was under attack.
Shater's own whereabouts were not immediately known. He is widely regarded as the strongest personality in the Islamist movement, but who was barred from running for president last year because he had been jailed under toppled ex-President Hosni Mubarak's authoritarian rule.
The incident occurred on a day when the armed forces issued an ultimatum to Islamist President Mohamed Mursi to agree within 48 hours on a power-sharing consensus with opposition parties or face more direct military intervention.
Generals from Egypt's powerful armed forces ruled Egypt for six decades until Mursi was elected last year after an uprising in 2011 ended Mubarak's rule.
Shater, a wealthy businessman and influential Brotherhood leader, was seen by many Egyptians as the shadow power behind Mursi's throne. He was hated by Mubarak who put him in jail for many years.
One of Shater's bodyguards was sentenced to one year in prison earlier this year for owning an illegal weapon. A security source said his other guards had "illegal arms and were a threat to Egypt's security".