Armenian Prime Minister Nikol Pashinyan has ignored an ultimatum by protesters who called on him to quit over a deal to end the fighting over Nagorno-Karabakh.
Pressured by the opposition parties, the Armenian parliament is expected to hold an emergency session on Mr Pashinyan’s political future.
Under the Russian-brokered deal, Azerbaijan keeps areas it has captured.
Hundreds of Russian peacekeepers are already deployed in the disputed area.
Turkey’s president said on Wednesday it had signed a deal with Russia to take part in “joint peace forces” to monitor the agreement.
Nagorno-Karabakh is an enclave internationally recognised as part of Azerbaijan but which has been controlled by ethnic Armenians since a 1994 truce.
In the past six weeks of fighting Azerbaijan has not only recaptured areas around the enclave but it has taken the key town of Shusha inside it too.
What is happening in Yerevan?
Thousands of protesters in Freedom Square in the Armenian capital have been chanting “Nikol is a traitor” and “Nikol, leave”, denouncing Mr Pashinyan’s acceptance of the peace deal with Azerbaijan. The demonstrators ignored martial law, under which rallies are banned.
The urged the prime minister to resign by midnight (21: 00 GMT).
According to reports by Armenian media, more than 100 people were detained and later released.
Hundreds of protesters then headed to parliament demanding the emergency session to push for the prime minister’s dismissal. Later reports said a special sitting had been convened at the request of two of the opposition parties.
Some of the protesters said the prime minister should have consulted the people before agreeing a peace deal, accusing him of breaching the constitution, a BBC correspondent in Yerevan reports.
Mr Pashinyan took office after leading a peaceful 2018 revolution in the post-Soviet state.
Under the terms of the agreement to end the conflict over Karabakh, Armenia has agreed to withdraw from parts of the enclave as well as adjacent areas that it seized from Azerbaijan in the 1990s.
Speaking via Facebook earlier, Mr Pashinyan insisted that if he had not agreed to halt the conflict, there would have been even greater losses – a comment backed up earlier by Karabakh’s ethnic Armenian leader Arayik Harutyunyan.
The prime minister said he had taken the “painful” decision following a “deep analysis of the military situation” to agree the deal, handing over three areas adjacent to Karabakh – Aghdam, Lachin and Kalbajar.
But after Shusha (Shushi in Armenian) inside Karabakh fell to Azerbaijan at the weekend, he said there was a risk of “total collapse” with thousands of Armenian soldiers being placed under siege and the enclave’s main city falling too. “We had a situation where Stepanakert was left defenceless.”
How will the deal be monitored?
Mr Pashinyan signed the deal with Russian President Vladimir Putin and Azerbaijani President Ilham Aliyev for 1,960 armed Russian peacekeepers to patrol the front line as well as the “Lachin corridor”, which links Nagorno-Karabakh to Armenia.
In a statement, Gen Sergei Rudskoy of the Russian General Staff said that 16 observation posts would be set up on the “line of contact” to prevent “illegal action” against civilians and escort convoys and cargo. More than 400 peacekeepers had already arrived and were in control of the Lachin corridor, he added.
Russia has a military alliance with Armenia as well as an army base, but it did not intervene during the conflict. It also has close ties with Azerbaijan and has sold weapons to both countries.
Turkey openly backed Azerbaijan during the conflict and President Recep Tayyip Erdogan said a Turkish-Russian control centre would be set up in the “liberated part of Azerbaijan” to monitor the ceasefire.