MS in Physics after Bachelors in Mechanical engineering
Changing fields from Engineering to Physics should not be a problem. I have several colleagues who have done this at various universities. Although, you would need to check with the specific universities you are interested in. If you are applying to universities in the United States you will likely need to take the GRE exam (some non-US universities will also consider GRE exam scores in the application process). As you are applying from India you may also need to take the TOEFL test. 

Your grades may hold you back since most high-ranked universities require an average over 75% for graduate studies. These are the University of Toronto minimum entrance grade equivalents from Indian universities:


  Bachelor's Degree Equivalent: Completion of BOTH a Bachelor's
  (three-year) degree and a Master's OR A Bachelor's degree requiring
  four years of study (e.g. BPharm, MBBS, BDS, BTech, BEng, or BArch)
  
  Mid-B Equivalent: 1st Class or 1st Div.; 7.5/10 from Indian Institutes
  of Technology; 8.0/10 from other institutions


However, good exam scores, relevant research experience, and good reference letters may help make up for the difference. It is worth discussing with a potential supervisor at your intended university who may be willing to support your application, as this can often help.