Egyptian riot police fired volleys of tear gas and locked down Tahrir Square and several other Cairo streets on Friday as clashes briefly broke out in a rare push by Islamist supporters of the ousted President Mohamed Morsy to take control of the iconic square.

With lines of armoured vehicles and barbed wire, troops sealed off the square and diverted traffic after the Muslim Brotherhood called on its supporters to march there.

In its statements, the Muslim Brotherhood called Tahrir Square “the heart of the revolution.” The Brotherhood appears to be angling to endure a crackdown that while painful also helps keep group cohesion under the pressure of a shared plight. It has publicly stuck to its most hard-line demands — the reinstatement of Mr. Morsy as President and the restoration of the Morsy-era constitution.

Some Brotherhood members recognise the possibility for that has passed. But the group uses the demands to energise its members and keep up street pressure as leverage in any eventual negotiations, which could bring concessions like easing the crackdown or releasing jailed members.

In an attempt to turn October 6, a national holiday — seen by Egyptians as a military victory in the 1973 war with Israel — into a milestone, the group called upon its supporters to converge into Tahrir Square in a show of force.

On Friday, protesters encircled security forces and army troops guarding the square from two main entrances, one near Egyptian museum and a second from the square’s southern edge. That prompted riot police to fire volleys of tear gas to send the demonstrators away. All the way to Tahrir Square, the protesters’ chants against the military sparked clashes with civilians believed to be supporters of the military. Shots were fired and rocks were thrown.

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