How can I study in Master in Theoretical Physics after completing engineering in electronics and communication in india?

All of the good colleges accept an engineering degree. As an engineer you are expected to have a strong mathematical background and a sound knowledge of basic physics. These two are sufficient to pursue a master's degree in physics. You can try to do the masters program at the IIT's. Admission is based on the http://gate.iitd.ac.in/jam/exam. I would also urge you to give the http://www.jest.org.in/ . It is required for admission in certain research institutes like http://www.hri.res.in/ , http://www.imsc.res.in/ , etc. Above all you can give the http://www.tifr.res.in/  entrance exam.

You can appear for the JEST and puruse Integrated MSc and PhD in Theoritical physics from any research institute such as IMSc, Chennai. They also allow BTech candidates.

I don't know anything about the system in India, so my advice may be useless; but if the options exist, why not either switch majors to Physics or finish your B.E. and then take a second degree in Physics before grad school?  It should only cost you a couple of extra years (which will look pretty minor when you're my age) and you will have an enviable preparation afterwards if you ever want to dabble in Experimental Physics!  Obviously I am ignoring all practical economic considerations in the short term, but your education is the best investment you will ever make, so it's worth stretching for if you can. 

You are same like me friend. Pay attention to what I say.Prepare for NET exams for admission in research centers and IIT's. If you want to do just MSc in Physics, you should take exams like GATE in Physics to do MSc from IIT's. If you want to do integrated phd, then take NET exams and JEST exams in Physics to get admissions in beautiful research centers like TIFR, HRI, IISER and all. You can choose any topic here to do your PhD's ranging from Quantum Physics to Particle Physics.Just remember. As you are Engineer, it will be hard for you to clear NET exams because you will have to focus on BSc Physics to clear this tough exam. Choose wisely otherwise you will be in great trouble.

why don't you give GRE physics exam!! there is no eligibility criteria for the exam and you can learn physics from the best colleges of the world

I have changed my stream successfully from engineering to physics. So it's quite possible. You have the following options.1) clear JEST (entry to 24 govt funded physics research institutes like TIFR, HRI, IUCAA, IISc etc) or JRF-NET or some institutes have their own entrance exams (INAT for IUCAA, GS exam for TIFR, NEST for NISER etc) and get direct PhD. NOT all institutes accept GATE scores. IIT's accept GATE for PhD. check institute website for information. However, you need advanced physics like statistical mechanics, advanced classical mechanics to clear.2)Few offer integrated PhD (like TIFR through GS entrance exam or various IISERs)3)Take up http://M.Sc at universities like the ones in Delhi, Pune, Hyderabad, Bangalore and then head for option 1. Almost all good universities have entrance exams for MSc. MSc in Indian universities is quite general and may not specialize you in theoretical physics.4) Try abroad.This option is tricky and its better to try decent universities instead for top ones, unless of course you are an extraordinary student. Masters is generally not offered in physics. So, better try for direct PhD. Its better to have physics projects or papers to increase chances of acceptance. In USA, you will need GRE, TOEFL and PGRE along with letters of recommendation and statement of purpose. In Europe generally it's very difficult for engineering students to get Physics masters or PhD. European universities need only IELTS along with good academics.