CAIRO — Egypt’s military rulers moved to consolidate power Friday on the eve of the presidential runoff election, shutting down the Islamist-led Parliament, locking out lawmakers and seizing the sole right to issue laws even after a new head of state takes office.

The generals effectively abandoned their previous pledge to cede power to a civilian government by the end of the month, prolonging the increasingly tortuous political transition after the ouster of Hosni Mubarak last year. The power play has also darkened the prospects that Egypt, the most populous Arab state and one that historically has had tremendous influence on the direction of the region, might quickly emerge as a model of democracy for the Middle East.

Their moves, predicated on a court ruling on Thursday and announced with little fanfare by the state news media, make it likely that whoever wins the presidential race will — at least at first — compete with the generals for power and influence. The military counsel also indicated through the official news media that it planned to issue a new interim constitution and potentially select its own panel to write a permanent charter. The generals have already sought permanent protections for their autonomy and political power.

The military’s power grab, which many critics have called a coup, presents a new obstacle to the ascent of the Muslim Brotherhood, the Islamist group that is Egypt’s largest political force. After 84 years in the shadows of secular monarchs and military autocrats, the Brotherhood dominated last fall’s parliamentary elections; it began drawing up plans to revise the Constitution and government ministries, and appeared poised to claim the presidency as well.