Are Hippopotamus' Herbivore, Carnivore or Omnivores?

Philippines
@raijin (10385)
March 5, 2007 1:58am CST
Hippopotamus. - A large african mammal that is mostly submerged on the rivers of Africa.
As to what alot of us knew, hippos are herbivores. They only eat plants, on to what I have also thought. But recently, I seen a documentary about their behaviors. I saw a man was viciously attacked by one of the hippos that almost caused his arm, and also his dear life. Hippos also attack and kill some of their own, maybe when food is scarce or maybe because other male hippos see them as a threat in the future. By this, cannibalism has already been proved. And the attack on humans only means that they have the tendency to change their moods, to which there's also a possibility that they are Omnivores. So how would we classify this animal? Has anyone else seen their documentary on, National Geographic Channel?