CAIRO — Egyptians on Thursday braced for the ninth weekend of protests against the military’s ouster of the country’s president as the looming possibility of Western airstrikes against Syria injected a new element of volatility onto the streets.

The degree of participation and violence at the protests expected on Friday will be a pivotal test of the effectiveness of the new government’s crackdown on the supporters of the ousted president, Mohamed Morsi, especially his Islamist allies in the Muslim Brotherhood.

Small protests in certain neighborhoods of Cairo, the capital, and larger demonstrations in other Egyptian cities have continued every night since Mr. Morsi’s ouster on July 3, despite an evening curfew, the suspension of due process and a wave of mass shootings and arrests by security forces that have decimated the Brotherhood. But the group’s decapitation as an organizing force has made the continuing protest movement harder to predict or control, potentially increasing the chances of violence.

Now the expectation of American-led airstrikes against Syria has added a new variable. The prospect of Western military action in the region is overwhelmingly unpopular here across the political spectrum, even if it is to punish President Bashar al-Assad’s government for the use of chemical weapons.