Below verse from ShriMad Bhagwat, infers, that when Lord Krishna left the planet, 1200 years of Kali Yuga had already passed.
“यदा देवर्षयः सप्त मघाषु बिचरन्तिहिं,
तदा प्रबृत्तस्तु कलि द्वादशार्द्द – शतात्मकः।“
Meaning: When the Sapt Rishis (‘Seven Sages’, a group of seven stars that forms part of Ursa Major Constellation) were in Magha Nakshatra, (corresponds to the time of Lord Shri Krishna’s leaving his mortal body), 1200 years of Kali Yuga had already passed by that time.
Later, after the death of King Parikshit, effects of Kali Yuga spread throughout the Universe. Under the influence of Kali Yuga people began falling to vices like greed, attachment, lust, anger, ego, prostitution, laziness, etc. Even though people had knowledge of Scriptures and Vedas, but they indulged in activities which are forbidden in the Scriptures & Vedas.
Those who, do not follow dharma, indulge in ‘adharmic’ activities, commit sins like killing animals, consume intoxicants, condemn the Vedas & deities, they are referred to as ‘mlecchas’.
Shri Jaidev Goswami, in ‘Dashavtar Stotram’, has written the following verse:
“म्लेच्छनीबह नीधने कलयसी करवालम्।
धूमकेतुमिव किमपि करालं,
केशब धृत, कल्कि शरीर,
जय जगदीश हरे।“
Meaning: Lord Kalki will incarnate and destroy mlecchas, with fierce force like that of a comet (Dhum-ketu).