Protesters have held rallies over recent days to denounce Sarkisian's efforts to remain in power as prime minister under a new parliamentary system of government.
Controversial constitutional amendments approved in 2015 have transferred governing powers from the presidency to the premier.
On Wednesday evening, more than 16,000 rallied in central Yerevan's Republic Square, vowing to mount a nationwide campaign of "civil disobedience" in opposition to the Kremlin-backed Sarkisian who was on Tuesday elected by parliament to the post of prime minister after a decade serving as president.
Pashinyan -- who earlier announced the "start of a peaceful velvet revolution" -- told Wednesday's rally that the protest movement's objective was to "change power" in Armenia through a nationwide campaign of "civil disobedience" and permanent sit-in protests inside government buildings.
The number of demonstrators dwindled on Wednesday, compared with Tuesday's rally of some 40,000 -- the largest Armenia has seen in years.
Protests began in Yerevan on Friday and have since spread to the country's second and third largest cities, Gyumri and Vanadzor.
On Monday police used stun grenades against protesters who tried to break through a barbed wire cordon to get to the parliament building.
Authorities said 46 people, including six police and opposition leader Pashinyan, sought medical help.
The country's new president, Armen Sarkisian, was sworn in last week but his powers will be weaker under the new system of government. The two men are not related.
A former military officer, Serzh Sarkisian, 63, has been in charge of the landlocked South Caucasus nation of 2.9 million people for a decade.
He also held the office of prime minister from 2007 to 2008.
After he was first elected in 2008, 10 people died and hundreds were injured in bloody clashes between police and supporters of the defeated opposition candidate.
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