More than 20,000 protesters rallied in Yerevan on Saturday, chanting "Nikol ,you traitor!" and "Nikol, go away!" and then marched to the prime minister's official residence.
"The seat of the prime minister of Armenia is currently being occupied by a political corpse," Artur Vanetsyan, the leader of the opposition party Homeland and the former head of the National Security Service, said at the protest rally.
Several priests of the Armenian Apostolic Church joined the protest, denouncing Pashinyan for allowing Azerbaijan to take over some holy sites.
Nagorno-Karabakh lies within Azerbaijan but has been under the control of ethnic Armenian forces backed by Armenia since a separatist war there ended in 1994. That conflict left not only Nagorno-Karabakh itself but large chunks of surrounding lands in Armenian hands.
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In 44 days of fighting that began on Sept. 27, Azerbaijan troops routed the Armenian forces and wedged deep into Nagorno-Karabakh, forcing Armenia to accept the Nov. 10 peace deal that saw the return to Azerbaijan of a significant part of the separatist region. It also obliged Armenia to hand over all of the areas it held outside Nagorno-Karabakh.
Azerbaijan completed reclaiming those territories on Tuesday when it took over the Lachin region located between the Nagorno-Karabakh and Armenia.
Armenian opposition leaders hold Pashinyan responsible for failing to negotiate an earlier end to the hostilities at terms that could have been more beneficial for Armenia. They have emphasized, however, that the opposition wasn't pushing for the annulment of the peace deal.
Veteran politician Vazgen Manukyan, whom 17 opposition parties have nominated as their candidate for prime minister, said at Saturday's rally that his transition government would seek to renegotiate some vague aspects of the Nov. 10 peace deal. Manukyan, 71, served as prime minister in 1990-91, when Armenia was part of the Soviet Union and later served as defense minister during the separatist war.
Armenia's Health Ministry said Wednesday that at least 2,718 Armenian servicemen were killed in the latest fighting. At least 55 Armenian civilians also were killed.
Azerbaijan said this week that 2,783 troops of its were killed and more than 100 were still missing.
The government said 94 of its civilians also were killed and more than 400 were wounded.
Azerbaijan celebrated the end of fighting as a national triumph, and President Ilham Aliyev established a new Nov. 8 national holiday called Victory Day to commemorate the event.
Azerbaijan's Defense Ministry said it will conduct a military parade next Thursday involving 3,000 troops and 150 military vehicles. It said the show will also feature trophy weapons seized from the Armenian forces.
Turkish President Recep Tayyip Erdogan is scheduled to visit Azerbaijan that day. Turkey has strongly backed its ally and used the hostilities to expand its clout in the region.
Earlier this week, Russian and Turkish military officials signed documents to set up a joint monitoring center to ensure the fulfillment of the peace deal.
Russia deployed nearly 2,000 peacekeepers for at least five years to monitor the peace deal and to facilitate the return of refugees. The Russian troops will also ensure safe transit between Nagorno-Karabakh and Armenia across the Lachin region.
Associated Press writers Vladimir Isachenkov in Moscow and Aida Sultanova in London contributed to this report.