Mahsa Alimardani, the associate director of WITNESS, an international nonprofit organization, estimates over 3,900 people have been killed and 24,000 arrested amid the 12-day blackout. ––– Subscribe to ABC News on YouTube: https://abcnews.visitlink.me/59aJ1G Watch 24/7 coverage of breaking news and live events on ABC News Live: https://www.youtube.com/watch?v=gN0PZCe-kwQ&ab_channel=ABCNews Watch full episodes of World News Tonight with David Muir here: https://youtube.com/playlist?list=