Can I start a Namaz in a room where there is non mehram person? does the namaz become invalid if i pray in the room while non mahram is there? Please give me reference also as that will be more helpful when i tell the answer to others..thanks alot.

Yes you can offer your namaz where there are non mahram but you should not recite loudly and the strongest proof in this concern is Hajj season and Holy Shrine of our Aimmah (a.s) where woman offer their prayer while there are thousands of non mahram are present.