NICOSIA, July 20 (Reuters) - Greek and Turkish Cypriots marked on Sunday the 51st anniversary of Turkey's 1974 invasion of Cyprus, an event that split the island and remains a source of tension between NATO partners Greece and Turkey.

Air raid sirens sounded across the southern Greek Cypriot-populated parts of Cyprus at 5:30 a.m. (0230 GMT), the exact time when Turkish troops landed on the northern coast in a military intervention triggered by a brief Greece-inspired coup.