First I will write about destructive evil Authority and the need to challenge all forms of authority, including a parent's, a facilitator's, and (especially) every police officer's authority. Challenging authority is obviously necessary and important, as well as a tenant of Anarchism. If there was no one to challenge authority life would be a slave-like misery (worse than today, I mean). For some, challenging authority means keeping those in power in check and assuring he rule of law (this includes challenging written law and refers to the ethical humanistic law that resides within us and our culture). For an Anarchist, or at least me, challenging authority also means finding out which institutions of authority can legitimate their existence and determines whether they should be altered or destroyed.
If an authority can make a case for its legitimacy, such as a parent (or any member of society) preventing a child (or any member of society) from using heroine, or a facilitator needed for any discussion to provide any sense of clarity (facilitators really don't have that much power), or a patrol of common folk to make the neighborhood secure against pigs.
Anarchists don't try to hijack events (depending on whether the anarchist is one of those closet commie assholes that wants to control everything...) when another has already organized everything (asking if they could help is nice though). Organizers are given an authority ver creating the unfolding events, but they are legitimate because they fulfill a function necessary to the movement (transparency when possible is moral though). just as a parent or a facilitator performs a function necessary to society.
Then there are people with authority from subjects like knowledge/expertise over mechanics, philosophy, current events, even cooking. These people who hold these skills have a predominant authority when a situation demanding these skills arises. It is pressured that these people explain and share these skills (teach them/distribute knowledge) to others. see: Ithaca Free Skool and Skill Shares
For example, should a group discover a fresh carcass, the cook and the hide-tanner (let's pretend they're meat freegans) would thus become the leaders of the following actions, cooking he eat and tanning the hide. This an Anarchist takes a somewhat tribalistic approach. However, the rest of the group would be involved in the actions, giving them insight into those skills, making the entire group cooks and hide-tanners, eliminating any division of labor. In this way, the "tribe" becomes informed of the variety of skills needed (excluding particular situations where the specifics are important, like medicine, some science, and engineering), limiting the need for authority.
Anarchists, from challenging authority, find most forms of authority coercive, power-hungry, evil, illegitimate. Therefore Anarchists seek to destroy the institution of authority and replace it (or not) with a better institution. Some authorities, such as full-time representatives are not legitimate and therefore should not exists. Police definitely fall into this category.
To quickly sum up and dispel them easily, Noam Chomsky says that should your child run into the street with oncoming traffic, should you not use forceful coercion, grabbing them (authority) to stop them?