SB 1.15.36

yadā mukundo bhagavān imāṁ mahīṁ
 jahau sva-tanvā śravaṇīya-sat-kathaḥ
tadāhar evāpratibuddha-cetasām
 abhadra-hetuḥ kalir anvavartata
Synonyms: 
yadā — when; mukundaḥ — Lord Kṛṣṇa; bhagavān — the Personality of Godhead; imām — this; mahīm — earth; jahau — left; sva-tanvā — with His selfsame body; śravaṇīya-sat-kathaḥ — hearing about Him is worthwhile; tadā — at that time; ahaḥ eva — from the very day; aprati-buddha-cetasām — of those whose minds are not sufficiently developed; abhadra-hetuh — cause of all ill fortune; kaliḥ anvavartata — Kali fully manifested.
Translation: 
When the Personality of Godhead, Lord Kṛṣṇa, left this earthly planet in His selfsame form, from that very day Kali, who had already partially appeared, became fully manifest to create inauspicious conditions for those who are endowed with a poor fund of knowledge.
Purport: 

The influence of Kali can be enforced only upon those who are not fully developed in God consciousness. One can neutralize the effects of Kali by keeping oneself fully under the supreme care of the Personality of Godhead. The Age of Kali ensued just after the Battle of Kurukṣetra, but it could not exert its influence because of the presence of the Lord. The Lord, however, left this earthly planet in His own transcendental body, and as soon as He left, the symptoms of the Kali-yuga, as were envisioned by Mahārāja Yudhiṣṭhira prior to Arjuna’s arrival from Dvārakā, began to manifest, and Mahārāja Yudhiṣṭhira rightly conjectured on the departure of the Lord from the earth. As we have already explained, the Lord left our sight just as when the sun sets it is out of our sight.