Thousands attended angry protests in Baghdad and southern Iraq Saturday, grieving but defiant after 20 of them were killed in an attack the previous day that demonstrators described as "slaughter". The dramatic developments have threatened to derail the anti-government rallies rocking Iraq since October, the largest and deadliest grassroots movement in decades. Late Friday, at least 20 protesters were killed or sustained wounds that later proved fatal, while dozens more were injured, when unidentified gunmen attacked a large building where protesters had camped out for weeks, medics said.