What to do if bullied by students?
In short, the behaviour of your students is not acceptable - no matter how good or bad your teaching was. 

If something like this were to happen at my university, I as vice dean for teaching would invite the whole group for a face-to-face meeting and explain to them how adults should solve problems. 

In fact, this is something you should aim for, since the problem will persist and affect others as well, since the students' behaviour will not change. 

We had good success with early feedback opportunities (not just the end term evaluation), micro-evaluations (anonymous feedback which can be a one-liner) which are discussed with the whole group, an honest open door policy, and with involving student organizations and representatives early on. Some course formats have to be explained to students over and over again... This might be helpful in the future, but won't help for now.

If you are still teaching this course, I'd suggest an intervention (if possible with the help of another neutral professor) where you address the problem in front of the class. If possible, summon an extra meeting which does not take place in your usual teaching room. 
For such a setting, I would suggest just asking the students about feedback regarding your lecture, but ask them to follow commonly accepted feedback rules. Afterwards, tell them what you took as the take home message from the feedback.

Afterwards, you should tell them that you prefer direct communication about any issues.

In the best case, they should be ashamed afterwards...