Democracy gives equal weight to the decisions and views of an individual in government; whether they're illiterate racist mountain people, or Ivy League economics professors. Marxism/Marxists/"Communists" aim to do away with government altogether; that's the crux of what I'm going to try to explain.

Marx believed democracy would lead to socialism (remember, Marx didn't see communism as a system opposed to capitalism, but as the inevitable progression following economic disparity and rapid technological advances of capitalism), but would inherently lead to inequality as Democracies and governments in general (and the masses with equal voting rights) are too easily swayed by the ruling classes (the bourgeoisie). Democracy might lead to equality in weight of a vote, but it does nothing to ensure equality in what that vote actually does. It also creates a system where votes can be bought, influenced, and decided by the rich and ruling classes.

Consider two wolves and a sheep voting on dinner; this is why America is a Republic rather than a Democracy. A Marxist would characterize this dilemma as a three sheep living with a wolf who controls all the grain, using grain to bribe two sheep to vote on a third sheep for dinner, in exchange for grain and their own safety. (at least until tomorrow, when the wolf would bribe one sheep to turn on the other)

Lenin, in "The State and Revolution" characterized Democracy as:

"The oppressed are allowed once every few years to decide which particular representatives of the oppressing class shall represent and repress them in parliament."

Even if someone from the oppressed classes (proletariat) won an election in a democratic capitalist state, they cannot function or rule without the cooperation of the economy; which is controlled by the bourgeoisie via the means of production. Eventually the same inequalities would surface, necessitating another revolution.

Communist countries are and were dictatorships, but "Communism" in a marxist utopia would be actually abolishing private property and creating a "dictatorship of the proletariat"; ie a society where the proletariat and majority demographic holds political power- which is a stepping stone-, eventually leading to the proletariat doing away with classes and state altogether.

In a "communist utopia", you would not have a government. This hasn't happened yet (just like America isn't a "democratic utopia"), but Marx saw it as an inevitable evolution over time.

I suppose the communist revolutions in Russia could be seen as the "dictatorship of the proletariat" stage; at best. Marx might have said that a government and movement like this was doomed to fail, as a true marxist revolution would need to be worldwide, or at least not when it had to compete with a capitalist geopolitical foe. Having major political foes led Stalinists to use the dictatorship as a means of consolidating political power; not to abolish political power and a Russian country altogether. This is the biggest hurdle for "communism"; using the government and political power to do away with the government is exceedingly tricky; especially when you have other governments in the world competing with your soon-to-be-powerless government. Marx would be the first to tell you that a communist society has to exist in a vacuum; via a worldwide revolution.

As Marx said in his "Critique of the Gotha Program":

Between capitalist and communist society there lies the period of the revolutionary transformation of the one into the other. Corresponding to this is also a political transition period in which the state can be nothing, but the revolutionary dictatorship of the proletariat.

Unfortunately for Marx, its exceedingly difficult to have an organized revolution where the revolutionaries are bent on giving up power and eliminating government altogether the moment the revolution is over.

"The master's tools won't dismantle the master's house"; government will never kill government; a true revolution according to Marx has to come not from a political party, movement, or person but from a worldwide proletarian movement bent on eliminating property and class.

As Orwell put it in "1984":

Power is not a means; it is an end. One does not establish a dictatorship in order to safeguard a revolution; one makes the revolution in order to establish the dictatorship. The object of persecution is persecution. The object of torture is torture. The object of power is power.

Just a reminder to people considering to post: historical examples only please, and preferably well sourced. Not 'your opinion as a communist', not 'this is what I think is wrong with democracy'.

One philosophical point, followed by some brass tacks:

"Communist" countries (that is, those one-party states run by a Communist party), didn't see themselves as fully communist. Rather, they self-identified as socialist. The party was the vanguard leading the way to the stateless, egalitarian utopia that was to be communism (insert old Soviet joke about Communism being on the horizon here).

There are minor examples of communists and Soviet-leaning socialist parties winning fair elections - Europe has a long history of Socialist parties (though Labour, SPD and Swedish communists were never Soviet-oriented). In the Americas, Arbenz in Guatamala, Allende in Chile, and Chavez in Venezuela came to power through elections. Plus, it happened many times in decolonized Africa (and I'm sure I'm missing many examples).

The US/western response (and some would say overreaction) to these victories (which has parallels in US responses to Islamist victories in Palestinian and Egyptian elections) is that fear that communists would only have to win elections once and then institute a one-party totalitarian state.

The reasons for this fear are both practical and theoretical. Post-WWII Central and Eastern Europe had (some) fair elections in which socialist parties won and quickly dismantled the multiparty states that had been setup (and then quickly eliminated private enterprise and unaffiliated civic organizations).

The theoretical rationale/reasoning for this comes from both capitalist and socialist sources. Capitalists, like Hayek, believed that in order to justify and maintain their control over the economy, socialist states would have to extend their control into other facets of like, eliminated Freedom of Speech and Association and, naturally, disallowing all but the most minor forms of free enterprise. Socialist theorists thought it a necessity that all members of a socialist state MUST be contributing toward the common goal - the eventual Communist utopia.

Many, if not most, communist groups throughout history have been pro-democracy in theory and practice. Marx went back and forth on the subject, but later in life generally supported the democratic process.

However, revolutions rarely lead to democracy. Homicidal strongmen tend to do better in civil wars than reasonable democrats. In countries where communists came to power through brutal knock-down, drag out fights with repressive regimes, democratic forces were always marginalized and violent, authoritarian groups seized power. Since authoritarian communist groups seized control of important, powerful countries like China and Russia they were able to export this ideology.

Communist groups which have evolved in less repressive situations (Euro-communist groups for instance) are almost always pro-democracy