Your problem is that Gare du Nord and Gare de l’Est are right next door to each other. Luckily you are headed west from Gare du Nord, which itself is west of Gare de l’Est, while the protesters will be headed southeast from Gare de l’Est towards Gare de Lyon.
However it could make things a mess even at Gare du Nord and surrounding areas. Make sure to arm yourself with patience knowing that you may take a while to get out of there —metro options are limited but at least on Saturdays they have been operating what they have been operating between 1 pm and 6 pm. Taxis could be problematic if the roads are congested.
This evening be sure to read Andrew’s daily update post, which will detail what metro lines will be available Saturday.
Once you make it to the 16th, you should be okay.