CAIRO — Egypt’s rival Islamist groups flexed their political muscle on Friday, drawing thousands of supporters from across the country to Tahrir Square to protest the presidential campaign of the longtime spy chief under the government of President Hosni Mubarak.

Islamists and secularists alike have viewed the unexpected entry into the presidential race of the candidate, Omar Suleiman, as a potentially existential threat to the country’s year-old revolution and fitful move toward democracy.

Protesters marched to the square from neighborhoods across the capital and rode in from the countryside in a fleet of buses provided by the Freedom and Justice Party, the political arm of the Muslim Brotherhood. The crowd also drew heavily from the ultra-conservative Salafi religious movement as well as those who could be called Islamist political independents — people who support the Islamist trend but have not decided which party to support.

“Down with Omar Suleiman!” the crowd chanted. “Down with the Zionist candidate! Down with military rule!”