Armenia prevented an assassination attempt on Prime Minister Nikol Pashinyan and the seizure of power by a group of former officials, the National Security Service (NSS) said on Saturday.
Mr Pashinyan had come under pressure with thousands of demonstrators demanding he resign over a ceasefire that secured territorial advances for Azerbaijan in Nagorno-Karabakh after six weeks of fighting.
The NSS said its former head, Artur Vanetsyan, the former head of the Republican Party parliamentary faction Vahram Baghdasaryan and war volunteer Ashot Minasyan were under arrest.
"The suspects were planning to illegally usurp power by murdering the prime minister and there were already potential candidates being discussed to replace him," the NSS said.
Mr Pashinyan saidlast week that he had no choice but to sign the agreement to prevent further territorial losses.
He said he was taking personal responsibility for the setbacks, but rejected calls to step down.
The ceasefire halted military action in and around Nagorno-Karabakh, an enclave internationally recognised as part of Azerbaijan but populated by ethnic Armenians.
Under the agreement, 2,000 Russian peacekeeping troops are being deployed to the region.
Since the early 1990s, ethnic Armenians had held military control over all of Nagorno-Karabakh and substantial areas of Azeri territory surrounding it. They have now lost much of the enclave as well as the surrounding territory.
After the 1991 collapse of the Soviet Union, Armenian separatists seized the land in a move supported by Yerevan.
An ensuing war left 30,000 dead and hundreds of thousands homeless.
Despite a ceasefire mediated in 1994 by Russia, the US and France, peace negotiations struggle to move forward and fighting erupts frequently.
Russia, which maintains close ties with Armenia, is the major power broker in the region. It leads the Collective Security Treaty Organisation military alliance of ex-Soviet countries that includes Armenia.
Yerevan relies on Russian support and military guarantees because its defence budget is overshadowed by Azerbaijan's spending on arms.
Azerbaijan has recently begun leveraging oil revenue as part of a bid to overhaul its image in the West.
Baku has invested in massive sponsorship deals including with the Euro 2020 football championship, which was postponed because of the coronavirus pandemic.