The attackers, mostly men in their early 20s, used rocks to chip away at the large foundation stone and sprayed it with red graffiti denouncing ousted President Mohammed Morsi and also Gen. Abdel-Fattah el-Sissi, the military chief who removed him in July after days of mass protests demanding that the Islamist leader step down.
The attack underscored the deep scars left by the political turmoil in Egypt since Hosni Mubarak's ouster in 2011, with revolutionary groups feeling betrayed by successive governments whose main failures, in their view, was the inability to dismantle the Mubarak regime and ensure retribution for the hundreds of protesters killed at the hands of police and soldiers since 2011.
They say the military-backed interim government, which was brought to power after the July coup that ousted Morsi is seeking to impose its control over what they see as an intrinsically anti-authoritarian space.
The pre-dawn attack today came just hours after military-backed Interim Prime Minister Hazem el-Biblawi inaugurated the foundation in a ceremony held amid tight security.
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All entrances to the square were sealed off by security forces and armoured personnel carriers, which caused hours of traffic congestion in Cairo.
The clashes in Mohammed Mahmoud street killed at least 45 people. Rallies are also expected later in the day amid fears of more unrest and violence.