"人民" is political word, and try to express some positive impression (for the government) of people, . "民众" is neutral, sometimes negative.
You can see in Chinese news, if they call some people "人民", they will usually say how good they are , or they are doing something right. And if they call some people "民众", normally they will say they are doing something not so good(e.g. fighting).
The world "人民” is abusing, or just like a political slogan. Actually, we don't know who belongs to "人民" in China. But I know most of the people are "屁民“