YEREVAN, Armenia (AP) — The abrupt resignation of Armenian Prime Minister Serzh Sargsyan after two weeks of protests against his rule has caught the opposition off guard: The protesters had focused on driving out what they consider a corrupt elite, and seem to lack the structure or the political platform to replace it.

Waving the Armenian tricolor and chanting their leader's name, some 10,000 opposition supporters marched on Tuesday with protest leader Nikol Pashinian to a hilltop memorial complex in Yerevan, the capital of this Caucasus Mountains country, to mark the massacre of 1.5 million A...