Sometimes when mass protests are ongoing, the protesters set some rubber tires on fire so that they produce smoke and unpleasant smell. But what's the purpose of doing so and why it is considered serious action by the government?
Burning tires produces thick black smoke that can be used as a signal to other protestors, and provides a means of communication that a government can't easily disrupt. An organized protest that can communicate is far more dangerous than a general unruly mob. The Smoke also makes it harder for law enforcement to use helicopters to gather intelligence and monitor the situation. Burning tires makes almost every aspect of suppressing a protest harder.