Thousands of anti-government protesters who gathered in Tehran Sunday were met by Iranian security forces’ open fire, leaving at least four dead in the country’s bloodiest dispute in months, witnesses say. The demonstrators chanted, “Death to the dictator,” rejecting official warnings about protesting Sunday. The day marks the religious observance of Ashura, which commemorates the death of a beloved Shiite saint. The demonstrations turned violent after security forces reportedly attempted to disband the gathering with tear gas and warning shots aimed into the air. When those efforts seemingly failed, they then reportedly turned their guns on the protesters, opening fire and killing at least three, according to witnesses and opposition Web sites. One of the victims, onlookers say, was an elderly man who’d been shot in the head.