A large march of striking workers just marched past my office.
There has been some violence in the past few marches but these are usually on a small scale and directed at those workers who elected not to strike. Usually marches are fairly harmless, even if they look aggressive and scary.
The question for a security professional is - how does one deal with a march that impacts business. In my opinion, besides electricity rolling blackouts and the winter sickness cycle marches are the most likely threat to business continuity that Johannesburg faces.
If you are a small business based in Johannesburg or any city centre then you should at least make sure that you have a business continuity plan. Make sure that you have backups stored away from your offices, a way to restore them to a separate location and a safe separate location that you can work from. This may be a bit of an issue for a small business (in which case lots of planning is needed . And don't forget to think out of the box, too!) but for a micro or mini enterprise it may be worth working from home or for the more adventurous - a coffee shop for the time that the march is on.
This is not a complete solution; each business needs to assess business continuity for themselves. Just remember that the first rule of Business Continuity is that the safety of all the employees comes before the health of the business.