Harassment from landlord

I am living in a rented house in Delhi which I going to leave soon. There is no legal rent agreement between us. I pay 1month advance rent for it. But when I told my landlord on 20th of this month that I am going to leave at the end of this month, he is demanding for 15 days security amount though I have paid the rent for this month. He has harassed me even beaten me when I disagreed to pay the amount. He has taken few of my valuable belongings as security, until I pay that amount (15days security)which is very less than the value of my belongings he will not release those things. Kindly suggest me a legal advice so that I can give him a proper lesson. This landlord having 2 such houses, earning more than 2.5L/month without giving any tax for this earnings.