The scale of the opposition protests — and the government crackdown — was unclear early Wednesday, but wire services and state-controlled media reported police using tear gas and clashing violently with protesters. Iran has largely banned media coverage of non-sanctioned rallies and protests, and international newswires with reporters in the country must rely on eyewitness accounts.

Still, initial reports suggest large antiregime protests erupted in several locations across Tehran. Large crowds of demonstrators gathered in central districts of the capital near the former U.S. embassy, where an annual pro-government rally was also taking place, according to eyewitnesses and video posted on Iranian websites. …

At one point, one crowd of protesters turned its message toward the American President Barack Obama, chanting, “Obama, Obama, You are either with us or with them.”

The American president has tried to make it clear.