Police have fired tear gas as clashes erupted during rallies by hundreds of supporters of Egypt's Islamist president in front of government buildings in the heart of Cairo.

Security officials and witnesses say the fighting broke out Tuesday when the Muslim Brotherhood, which is demanding that President Mohammed Morsi be reinstated, tried to break into the Interior Ministry where they said members are being held.

Protesters threw rocks at police trying to stop them, prompting the tear gas as local residents joined security forces and began throwing rocks at the Brotherhood supporters.

The violence came a day after Egypt's military-backed leadership postponed a plan to besiege two protest camps occupied by Morsi's supporters to avoid bloodshed.

Meanwhile, Egypt's new leaders named 20 new provincial governors on Tuesday, in a move to enforce their authority by removing all Muslim Brotherhood members previously installed in the posts by the former government.

The shake-up came as supporters of ousted President Mohammed Morsi reinforced their 6-week-old sit-ins in the capital and rallied for more protests across the country to demand his reinstatement.

The Islamist Brotherhood is rejecting talks with the new government, much less participation in the post-Morsi transition. They say the military-backed new leadership is not serious about reconciliation or meeting a key demand of protesters to release top Brotherhood leaders charged with inciting violence.

Seven of the new governors, appointed by interim President Adly Mansour, come from the military. In line with tradition, most of them took up posts in border governorates. But two of them were sent to the Mediterranean cities of Port Said and Alexandria, where clashes between pro and anti-Morsi protesters have turned violent, and a third was given the ancient city of Luxor, a prized tourist destination.

The changes to the administration of Egypt's 27 provinces meant the official removal 10 Brotherhood governors appointed under Morsi. Many members of the group who worked in government under him had already left their positions to join the sit-ins in Cairo.

Shortly before mass protests that led to his ouster, Morsi appointed 17 new governors in a move that critics said was aimed at solidifying Islamist power. The appointments had added to the Brotherhood's already entrenched grip on the executive and legislative branches at the time.