For a banned demonstration called off by the opposition candidate, this looks pretty big. New York Times says “hundreds of thousands,” i.e., more than came to see Obama in Berlin.

Government apparently continuing to try to crack down on media. Much information still coming out, but verification in the old media sense is difficult, and the situation evolving very quickly, as darkness has just fallen in Tehran.

NYT blog is forwarding reports of shooting in Azadi Square. If true, and if there is more, that would certainly change the game. Does the opposition have enough people power? How much force will the government use? Those are tonight’s questions.

Tehran tense, says CNN. Unrest challenges Iran’s republic, says the BBC headline writer, choosing understatement. The reporter, Jon Leyne, is less restrained: “As demonstrations against the Iranian election result continue, the situation in Tehran is becoming unpredictable and potentially explosive.”

The story got close to a third of Germany’s main news broadcast last night, too, with heavy emphasis on the government’s efforts to keep international reporters away from any stories. ARD filmed from the correspondent’s office, and told how revolutionary militias had forced their way in earlier, threatened everyone and abducted one of their technicians. According to the report, international journalists are also being regularly detained by government forces, but usually released after a few hours.

Despite these efforts, there’s lots of news getting out of the country. In addition to all of the media, here is a list of English-language Twitter feeds coming from Iran. (Thanks, Tobias.)

We’ve seen some of this story before, but the ending is far from certain. Is it like Kiev, where electoral fraud brought people out for long enough to force change? Is it like Belarus, where the opposition stayed intimidated? Is it like China, where the powers that prevented change with a massacre? This morning, all of these seem possible.

But with the Khameini calling Ahmadinejad’s alleged victory “a divine miracle”, the power structure looks to be lining up behind the status quo. The government is not shrinking from using violence, and with non-uniformed “militias” and “activists” committing much of the violence — what would be criminal in other countries — this looks like a severe test for Moussavi supporters. Do they have countervailing powers? Any police or militias or military going over to the opposition? Absent something along those lines, change is unlikely. At least not now.

(Just want add that Google’s News page is fantastic. Quick links to full coverage of articles, blogs, local sources, images, quotes and videos. In decades past, presidents were probably not so well informed.)