In a phrase where the subject is "*mass of people*", should I use "*its*"?

e.g.

>*A mass of people and its flag.*

***Mass*** is an abstract word, so it sounds strange to me to use ***their***; but I'm not sure.


EDIT:

Seems that according notional agreement I should use ***their***

but

If the mass of people act like a cohesive body, e.g. a "*mass of people is/are united in its/their flag*"... Is this situational agreement and is the use of plural forms wrong? 


Could you help me?