Hi,
Somebody please clarify my below mentioned doubts with regard to reference letters:
1. Are these letters required to be obtained only from HR departments of previous organizations for whom I have worked for previously ?
Ideally, yes. But this is not practical for everybody.
2. or letters from colleagues of those organization will suffice?
Should work. Be sure to get their ID card/Business card copy along with the letter.
3. HR departments of few companies are reluctant to issue any reference letter except for the relieving letter they issue during separation with those companies. What is the solution in that case?
Precisely. My company is like this. In this case, if there is nothing you can do, you can ask your manager or a senior to provide a letter in the required format. You can ask him/her to mention his/her contact details in the letter. Here too, be sure to attach a copy of the business card/ ID card of the person giving the reference. Also, be sure to show proof that you genuinely tried to get the letter in the required format from the HR but they denied it (You can submit email exchanges).
4. Does lack of reference letters from HR departments will have adverse impact on the application?
Since not everyone will be able to get such a letter, IMO, no, this will not have an impact on the application but its a drawback for sure and the onus of providing additional documents to support your case lies on you.
6. How to cover the reference part w.r.t. current organization as it will be imprudent to ask my reporting manager to give me a reference letter? What other documents (appointment letters, confirmation letters, salary increment letters etc..) will be considered as substitutes for reference letters.
There is no "substitute" for a reference letter, per say. You need a letter. The thing is, you can get it from a trusted colleague if required (if you cannot get it from the HR department). All the documents you have mentioned are supporting documents and not substitutes as such.