Andrew Sullivan, The Guardian, CNN, Huffington Post, and several other news sources are reporting that there were massacres last night and today in Iran. Many reports are about one that occurred at Baharaestan Square during which hundreds of men armed with clubs emerged and beat demonstrators mercilessly. See Andrew Sullivan's post on this here; it includes a recording of a phone call from a woman who called CNN from Iran about the violence. Hundreds are reported to have been beaten and shot (some severely, some fatally) by the Rev Guard and Basij. You can also read the firsthand accounts here at the Guardian blog.
Hospital workers are providing many of the accounts, which the government has tried tirelessly to prevent them from doing, read one here at The Guardian.