When citizens get angry with their government or leader, they may revolt against the government. To do this, they may try to get to the leader and do him or her harm. The group is usually a very large group of citizens from this country and they may try to overthrow the government.
During this time, the country may impose Marshall Law which means that the government may exert control over the citizens and require them to act a certain way. There are other things that may lead to a government exerting Marshall law on its citizens. Another way may be because of an invasion from another country or a disaster. It is done in order protect the citizens and government.