Are not the same thing. That's just to get us off on the right foot. A leader is a person whose charisma helps them to guide a group of people in a direction they believe is desirable. Someone with authority uses their power to guide a group of people in a direction they believe is desirable. The key difference is, of course, power. If a person has the ability force a person to perform a certain act, or the ability to otherwise coerce them, than that person is an authoritarian. They may also be a leader, but they are definitely an authoritarian. A leader guides people by the infectious nature of their vision. People want to follow them, but they are perfectly free not to. This freedom must include the clause "with no negative repercussions applied by the leader or the group," otherwise the leader is also an authoritarian.
There is no small question as to whether there is such a thing as a leader exclusive of authority. After all, peer-pressure is a form of coercion. And, if someone really is a leader, than a small minority opposed may feel oppressed by the majority's agreement.
I would tend to believe that there are several factors that would help to prevent authoritarian behavior, at least for the most part. Firstly, if everyone in the group feels comfortable and unconditionally accepted then expressing a dissenting viewpoint becomes easier. Secondly, if the leader is not in a threatening position than people are less likely to feel threatened by the leader. Third, the leader needs to be willing to be wrong. The third seems odd, but, as long as the leader is willing to be wrong, the leader is inherently willing to accept other people's viewpoints. And that means that expressing an opposing opinion might ultimately change the leader's mind, which would help shift the group's direction. So if you are in disagreement, expressing that disagreement might not be wasteful or counter-productive.
An authoritarian can most readily be recognized by how they make their decisions. Sometimes there is little to no discussion, or the discussion begins with a foregone conclusion. Also, an authoritarian will guide the group with negative motivation. "Do what I say or you will be removed from the group," and such are common phrases. This leaves people in a bad position. They often feel that they are the only ones who disagree, and that they must comply or be exiled. Ultimately, this means that the only way to really oppose an authoritarian is with the support of a sizable percentage of the group. Usually an authoritarian will speak of betrayal in this case. A true leader would not, as a disagreement is never a betrayal unless there is an expectation of being followed and obeyed.