It was a government call. I grew up in USSR and know very well how those government "calls to volunteer" work in totalitarian regimes. Especially in a wartime country where even in peacetime they would kill people even just for being incorrectly dressed.

Anyway, as i said in the other comment, it is actually not that important how all those people got there. The key thing here is that it was a deliberate government act of human shield creation.

what a coincidence i too grew up in USSR and my parents and friends were part of above mentioned human shield. And i can tell first hand that there was no coercion. just call to action.