You can do this in a variety of ways. If one has the money for it you can start bribing mayors and law enforcement officials to look the other way and gain influence. If there is a racist movement going on campaign in favor of that to feed off of those angry people. This is like how Hitler became Chancellor of Germany.
Another way is to start a revolution in the country against something that the citizens are very unhappy with. Declare yourself as the leader of the revolution. Make sure the revolution is a success. When the revolution is a success, you will be seen as a hero and instantly gain favor of the people. Once this happens make sure you remove all rivals (personal rivals, political rivals, so on) from the country and start centralizing your power. Start spending country funds on militarization and starve your people to intimidate them. Make your people worship you. Have pictures of you, the Great Leader, in the living rooms of every house by law. This is how the leaders of North Korea have done, and they are still worshipped by their people today.
