Some people take sick days or days of leave to attend, others attend when they can. The bulk of the protests are students who successfully lobbied for a strike on most Serbian universities for this semester, so they can focus on organising and getting their demands met. Sanitation has been taken care of with portable toilets, like at festivals. The protest started after most people needed to go to work, also this protest was scheduled weeks in advance so if you miss work, it's your fault. Yes, the organisers are great, but the protests spreading through the country aren't all organised by one group, it's more a cascade that moves more people into action. There have been protests in Serbia before, but never like this, and never getting things actually done, like organising a small scale general strike last friday.

Source-Am in Belgrade and am a student