Islamists in Egypt's capital rallied Saturday to support President Mohamed Morsi in what is emerging as a decisive battle with opposition forces in the country's messy political transition away from three decades of Hosni Mubarak's corrupt and undemocratic rule.

Tens of thousands of Muslim Brotherhood and ultraconservative Islamists marched in a counterdemonstration to an energized week-old protest across the Nile in Tahrir Square by opposition groups challenging Morsi's expanded powers. Islamists back the president for his religious ideology but say Morsi's central aim is to rid state institutions of the lingering influence of the Mubarak era.