Police in Chinese / Japanese...

警察 is the Chinese, Japanese Kanji, and old Korean Hanja title for police (or police officer).

Police cars all over China and Japan have these characters painted on the doors. Police officers' helmets and equipment are also emblazoned with these two characters.

公安 is the Chinese, Japanese Kanji, and old Korean Hanja title for (The Ministry of) Public Security. 公安 can also generally mean public safety, public security, or public welfare. It is a positive term in Japan, where some even name their daughters “Kouan” (this title).

In China, this is the kinder name for the PSB or Public Security Bureau. It's really the national police of China - occasionally brutal and seldom properly-trained or educated. Once in a while, you find a PSB officer who lives up to the title of 公安. Before the 1989 massacre, it was the PSB officers who refused to stop nor kill any of the protesting college students (so they're not all bad). The Chinese government had to call in soldiers from Inner Mongolia to kill thousands of protesters.