On this day 20 years ago in the tiny African nation of Rwanda, Hutu militias began an organized ethnic cleansing campaign that resulted in the slaughter of more than 800,000 Rwandans, primarily ethnic Tutsis, in less than two months, while the world stood by.

At a somber memorial ceremony in the nation's capital of Kigali, victims wailed and fainted as they recounted the horrors of those days, even two decades later. Leaders from all over the world attended, including the United States UN Ambassador Samantha Power. Her Pulitzer Prize-winning book "A Problem from Hell," is considered one of the most definitive accounts of the genocide. In it, Power details how the U.S. government and international community's slow response contributed to the atrocities.

"The U.S. government knew enough about the genocide early on to save lives, but passed up countless opportunities to intervene," Power wrote in a 2001 Atlantic Monthly article.

Former President Bill Clinton has called Rwanda one of his greatest regrets during his presidency, admitting that had the U.S. and the world intervened earlier, some 300,000 people might have been saved.

"I do feel a lifetime responsibility," he told ABC in 2008. "I feel like a lot of people … had something to do with it."

Human rights groups around the world have also used the anniversary as a time to reflect on the genocide, and the lessons learned since.

Doctors Without Borders, known by its French name Medicines' Sans Frontiers or MSF, released a detailed report on the group's response to the conflict 20 years ago. The assessment reveals harrowing accounts of decisions by MSF leadership to evacuate international staff after trying desperately to convince Rwandan custom agents to let local staff flee as well, knowing the decision to leave them meant certain slaughter.

"While the Rwandan case studies investigate MSF's own dynamics and operational challenges in response to the genocide, they also bring back all the human loss, and for MSF as an organization, the brutal loss of several hundred Rwandan colleagues," said Dr. Joanne Liu, MSF international president.

The Enough Project, an organization focused on genocide prevention and response, which works with celebrities such as George Clooney and Don Cheadle also released a report, arguing that mass atrocities in Africa will continue to occur until the underlying causes for violence are addressed.

One such case is the Central African Republic, which Ambassador Power will visit later this week. More than 2,000 people have been killed in ethnic-religious violence since December and at least 1.6 million people in need of urgent food aid. During a visit to the country yesterday, UN Secretary General Ban Ki Moon reportedly warned that CAR is on the verge of being another Rwanda if the world doesn't pay more attention.

"The international community failed the people of Rwanda 20 years ago. And we are at risk of not doing enough for the people of the CAR today," he said. "Atrocity crimes are being committed in this country."