Capitalism builds on competetion but favors behavior which eliminates competitors. This inner tension of capitalism makes it easily degrade into an authoritharian system. But it does not make it the same as one. Regulation is needed to maintain fair competetion which sounds paradoxical but is also a tension in the capitalism as such.
Democracies struggle with capitalism but they struggle much more with planned state. Struggle is in the nature of free agents of democracy, so it does not have to suggest incompatibility.