AP Photo

Iran’s opposition movement showed it may have staying power Wednesday as thousands of opposition protesters took to the streets alongside a regime-backed demonstration marking the 30th anniversary of the takeover of the U.S. Embassy in Tehran.

A multitude of security forces and plainclothes militias cracked down violently on the protesters, shooting tear gas and leading baton charges throughout central Tehran.

Students defied government warnings and the forced closure of some universities to gather at Tehran and Sharif universities, the Iranian capital’s main campuses. Riot police closed down the city center and assaulted groups wherever they gathered.

Grainy videos streaming out of the Iranian capital on YouTube and Facebook showed gas mask-wearing...