How to invest in / buy government bonds?

  1. what is the minimum amount one can invest ?

  2. minimum & maximum lock-in periods ?

  3. procedure to encash on / before maturity ?

  4. any other relevant info .

If your query is regarding Government Of India Savings Bonds (Taxable)

you can check answer here http://tradingqna.com/21285/which-online-company-provide-opportunity-invest-govt-bonds

 There is no maximum limit for investment. Bonds are issued at a minimum amount of Rs. 1000/- (face value) and in multiples thereof. and the Bonds shall be repayable on the expiration of 6 years from the date of issue.

the query is open to all kinds of bonds / securities central govt. ,state govt. , municipal etc .
taxable / non-taxable etc ,long-term,short-term etc ?

Does zerodha provide margin to trade on Central Government bonds like REC, NHAI etc

They aren’t present in the list of approved securities for getting collateral margin. This likely would be the reason for that -