Question

Are Marxists authoritarian?

Answer 1

Yes.

Answer 2

For the most part, yes.  Sometimes no or not necessarily, such as in the case of libertarian Marxists (council communists, autonomists, situationists, some left communists and some anti-leninist communists).  It might be better to assume yes unless they qualify their answer up front as "libertarian Marxist".

To some anarchists who don't want to participate in a more up to date discourse on the definition of dictatorship of the proletariat (considering it a Marxist discourse), this would qualify many libertarian Marxists as authoritarian.  A traditional anarchist view on dictatorship of the proletariat is that it encourages elite rule of a revolutionary mass implicitly and more often explicitly.  

Some libertarian Marxists would disagree, saying the dictatorship does not necessarily imply elite rule and the concept is more about an entire proletarian class acting as a forceful, anti-authoritarian power committing mass acts of direct action through revolution to destroy class society.  This is very much like some anarchist conceptions of revolution.

Other areas where libertarian Marxists might be authoritarian outside of continuing to use archaic language to describe their desires might be how they conceive the behavior of their political organizations, which many times act as vanguards.  And again during revolutionary situations, libertarian Marxists, who might be like anarchists in many ways outside of revolution will during revolution see a need to take on a mass leadership role, even "leadership through ideas".

"Leadership through ideas" is not solely a libertarian Marxist concept.  Many anarchist communists, like platformists or specificists tend to see this as a role they hold towards a movement for revolution.  Other social anarchists, like collectivists, anarchist syndicalists or synthesis anarchists, might assign this style of leadership role to their political organizations.  "Leadership through ideas" is often considered a quasi-Leninist or vanguardist concept.

Many libertarian Marxists reject "dictatorship of the proletariat", "leadership through ideas" and all styles of elite rule before, during and after a revolutionary situation occurs.  Due to the complicated nature of Marxism, most anarchists will assume authoritarianism from self-identifying Marxists.

For me personally, I don't fear use of dictatorship of the proletariat as a descriptive term in discussion or when reading particular authors I might give a grain of salt on the issue as to avoid falling into a stream of objections for semantic differences.  However, in analysis, theory and propaganda in contemporary times, the term is outdated with both "dictatorship" and "proletariat" having many other descriptive terms that sound less authoritarian and might describe an anti-authoritarian concept better, should they claim to be a libertarian of some sort.

The same problem could be said of calling yourself an explicit communist without any other qualifier attached to it.  A communist without any other qualifier (libertarian communist, anarchist communist, council communist, anti-state communist, left communist etc.) tends to be a Marxist-Leninist communist.

Answer 3

Not all of them, the better ones don't call themselves "Marxists"(in the same sense that the ol' moor himself declared that he wasn't a Marxist) but find his critique of capitalist political economy useful in understanding capitalism. Even among those who cling to the label there have been some pretty strictly libertarian currents throughout history that developed in opposition to the predominant reformist and Leninist varieties; the Dutch and German "left-communists" were the most influential in this regard despite their own failures and dogmatic idiosyncrasies.

Marx himself was a very creative and dynamic thinker throughout his life, but it's important to understand that not all Marxists take his writing as gospel or think of Marxism as a sort of closed-system of thought. At some times during his life he wrote better things than other times, he was a product of his environment and wasn't infallible. The better(non-authoritarian) Marxists tend to understand this in some regard or another. If you want to know if a specific Marxist individual or organization is authoritarian, then you can typically find that out by looking at how they conduct themselves: Do they seek power over others? Do they seek to represent and/or speak for others? Do they operate in a transparent and non-hierarchical manner?

Read and understand Marx for yourself, then have conversations with those you've got questions about. You can pretty much weed out most parties or Leninist organizations as authoritarians, but in the cases of individuals and non/anti-Leninist organizations you're going to have to determine that on a case-by-case basis.