Resistance was widespread, though it was divided between the communist National Liberation Front (EAM, and its military wing ELAS) and the non-communist National Republican Greek League (EDES). In each case, it was armed guerilla bands which harried German forces, disrupted communications, and mobilized Greek civilians against the occupation.

Franz Coetzee and Marilyn Shevin-Coetzee, The World in Flames: A World War II Sourcebook (Oxford UP, 2011), 184

Check out the British Reports on Greece 1934-1944 for a good primary source account of the situation in Greece at the time.

The Greek resistance was one of the fiercest of the occupied countries during its time. However, the reason it fell so quickly was due to the great tactics of the German paratroopers, having been the second time that Germany used these men, after the invasion of Norway. The Italian army were also involved in capturing the mainland of Greece, but not so much the surrounding islands. Crete, being an island, was obviously the last place in Greece to fall. Crete also was home to the larger pieces of resistance in Greece, compared to the mainland. As the ratio of German troops in Greece greatly differed from that of France and the Netherlands, the Greek resistance made much more of a local effect on the Germans than international. Hope this helps! Here's some more reading on the subject:http://www.ahistoryofgreece.com/worldwarII.htm