Opposition parties in Armenia on Monday staged protests to demand Prime Minister Nikol Pashinyan resign, accusing him of compromising under international pressure on the long-contested region of Nagorno-Karabakh.

Arch-foe Caucasus neighbours Armenia and Azerbaijan have been locked in a dispute since the 1990s over the mountainous enclave in Azerbaijan predominantly populated by ethnic Armenians.

Karabakh was at the centre of a six-week war in 2020 that claimed more than 6,500 lives before it ended with a Russian-brokered ceasefire agreement.

Opposition parties accuse Pashinyan of plans to give away all of Karabakh to Azerbaijan after he told lawmakers last month that the "international community calls on Armenia to scale down demands on Karabakh".

Waving Armenian and Karabakh flags and shouting demands for Pashinyan to step down, some 5,000 protesters marched on Monday evening in central Yerevan.

"We are launching a popular protest movement to force Pashinyan to resign," parliament vice speaker and opposition leader Ishkhan Saghatelyan told AFP ahead of the rally.

"He is a traitor, he has lied to the people," he said, accusing the 46-year-old leader of wanting to hand over the contested region to Azerbaijan. "He has no popular mandate to do so."

Saghatelyan said "protests will not stop until Pashinyan goes."

One of the demonstrators, 53-year-old dentist Hripsime Mkrtchyan, said: "Nikol must resign. His poor policy has led to territorial and human losses."

"Our people have never been in such a depressed mood. We don't see a light at the end of the tunnel."

Earlier in the morning, public transport was disrupted in Yerevan as small groups of protesters attempted to block traffic in the city centre.

Demonstrators wave national flags as they attend an opposition rally in Yerevan on May 1, 2022
Demonstrators wave national flags as they attend an opposition rally in Yerevan on May 1, 2022 AFP / Karen MINASYAN

Police intervened, briefly detaining dozens of protesters.

The Union of Journalists, a media advocacy group, criticised police tactics as heavy-handed, saying there were several instances of officers punching journalists covering the protests.

On Sunday, several thousand protesters rallied in central Yerevan to demand Pashinyan's resignation.

Under the Moscow-brokered deal, Armenia ceded swathes of territory it had controlled for decades and Russia deployed some 2,000 peacekeepers to oversee the truce.

The pact was seen in Armenia as a national humiliation and sparked weeks of anti-government protests, leading Pashinyan to call snap parliamentary polls which his party, Civil Contract, won last September.

US Secretary of State Antony Blinken, welcoming Armenian Foreign Minister Ararat Mirzoyan to Washington, praised Pashinyan for his leadership as he signed an accord on civil nuclear cooperation.

Blinken said he wanted to "express real appreciation for the vision and the courage and the flexibility that the prime minister and Armenia have been showing in this process working toward what we all want, which is a lasting peace".

In April, Pashinyan and Azerbaijani President Ilham Aliyev met for rare EU-mediated talks in Brussels after which they tasked their foreign ministers to "begin preparatory work for peace talks."

The meeting came after a flare-up in Karabakh on March 25 that saw Azerbaijan capture a strategic village in the area under the Russian peacekeepers' responsibility, killing three separatist troops.

Baku put forward in mid-March its set of framework proposals for the peace agreement that includes both sides' mutual recognition of territorial integrity, meaning Yerevan should agree on Karabakh being part of Azerbaijan.

Mirzoyan sparked controversy at home when he said -- commenting on the Azerbaijani proposal -- that for Yerevan "the Nagorno-Karabakh conflict is not a territorial issue, but a matter of rights" of the local ethnic Armenian population.

Ethnic Armenian separatists in Nagorno-Karabakh broke away from Azerbaijan when the Soviet Union collapsed in 1991. The ensuing conflicts claimed around 30,000 lives.