"In December 1963, the city became the scene of intercommunal riots and
clashes, as a result of Ankara's policy, which aimed at the self-enclavement
of Turkish Cypriots. The British Secretary for Commonwealth Relations,
Dunkan Sands, rushed to Cyprus in a bid to restore calm. A contact committee
of Greek Cypriots and Turkish Cypriots was set up and met at the British
High Commissioner's residence under Sand's chairmanship. At these meetings,
General Young, who with the help of British troops had undertaken to
supervise the ceasefire, marked in green color on Nicosia's map the
confrontation line that would serve as a buffer zone. Thus ths Green Line
came into being."

from http://www.kypros.org/Lefkosia/line.htm