In the 50s after the revolution, to justify the new regime on ideological grounds, Mao sanctioned freedom of speech against the state. But after a few years when the party failed to deliver some of its promises, criticism began to pile up, and the party responded with a massive crackdown of the witch-hunting and labour camp variety. This lasted through the cultural revolution of the 60s and 70s, and was enforced by the cult-of-personality campaign initiated by Mao's wife.
After Mao died and his rival Deng took over, there was quite a bit of freedom during the 80s in reaction to the economic reform and the end of cultural revolution, where pro-democracy activism accumulated that eventually led to the Tiananmen Square protest. The government again responded with a crackdown.
After the crackdown, government became more paranoid in the 90s, "patriotic education" were ramped up in public schools, though it paled in comparison to the Mao era (no more witch-hunting, correction camp, public humiliation, etc.).
Then in the 00s the internet became popular for the middle class, and cell phone for the peasants, but the government hasn't loosened its grip, so rather than ideologically-backed student activism, grassroot vigilantism were organized by regular folks against anything from corrupt local officials to animal cruelty cases. These usually come in the form of protests numbered upward of tens of thousands. To let off the steam in a controlled manner, the state sanctioned these protests (not the case before) as long as they are focused on the "local" bureaucrats (provincial and lower). In the mean time, the press are gradually given more freedom to cover these topics, as long as they are "localized" as well.
Overall, eventhough the state is using ever more sophisticated methods to maintain control, it's usually in response to greater pressure from the citizens, and the state seems to be losing ground.