Mr Pashinyan, who had been detained and then released from jail during the protests, called for snap elections to choose a new "people's prime minister". It marks a dramatic split from a corps of leaders who have run Armenia since the late 1990s, developing a reputation for corruption and cronyism.
Sporting a salt and pepper beard, a tan and a baseball cap, Armenia's rising political star channelled widespread desire for change into a broad protest movement.
The result amounts to a peaceful revolution in Armenia, a small nation of around 3 million people that borders Russia, Turkey and Iran.
Supporters say Mr Pashinyan, who was also imprisoned after opposition rallies in 2008 turned deadly, is among history's great peaceful revolutionaries. "The page of hatred should be turned", he said, adding: "May God help us".
Armenia's parliament has opened a session to elect a prime minister, for which opposition lawmaker and protest leader Nikol Pashinian is the only nominee.
The election of Pashinyan, a former newspaper editor who spent time in prison for fomenting unrest, marks a rupture with the cadre of rulers who have run Armenia since the late 1990s.
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On Monday evening thousands of Pashinyan supporters rallied in Yerevan's central square, waving tricolour national flags and chanting his name.
Prime Minister Nikol Pashinian faces an array of challenges, including a parliament dominated by the party he denounced as corrupt and how to resolve the volatile question of Nagorno-Karabakh, a region of Azerbaijan under the control of ethnic Armenian forces. Elections will not be rigged and vote bribes will not be handed out anymore, period! "However, if it turns out that they are ineffective, they will be discontinued", he said.
Pending fresh elections "Pashinyan must manoeuvre between the will of the people and the parliamentary ruling party that he does not belong to and which can not begin supporting him", he said.
Pashinyan's protest movement had accused Sarkisian of a blatant power grab. "That's it. Full stop", the newly elected prime minister said, reaffirming support for human rights protections and an end to corruption.
Council of Europe observers have said the referendum was marred by allegations of large-scale vote-buying and multiple voting, among other irregularities.
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