Police in Armenia detained three opposition leaders and nearly 200 protesters today, drawing a rebuke from the European Union after demonstrators demanded newly appointed Prime Minister Serzh Sarksyan quit.

Protesters accuse Mr Sarksyan, 63, of clinging to power after parliament made him prime minister this month following a stint of 10 years as president.

In the biggest political crisis in a decade, tens of thousands of his opponents have marched through the capital Yerevan, blocking streets and staging sit-ins.

The protests, though peaceful so far, threaten to destabilise a key Russian ally in a volatile region riven by a long low-level conflict between Armenia and Azerbaijan and would, if successful, be a rare example of people power delivering reform in the former Soviet Union.

Armenia protests
Armenian police detain an opposition supporter during a rally in Yerevan

Critics accuse Mr Sarksyan of ruling the South Caucasus nation of around three million people for too long, of being too close to Russia which has military bases inside Armenia, and of doing too little to root out corruption.

Mr Sarksyan says his country needs him and that his party enjoys large-scale popular support.

Under a revised constitution approved in a 2015 referendum, most state powers shifted to the prime minister while the presidency has become a largely ceremonial post.

Police said in a statement that opposition politician Nikol Pashinyan and two other politicians had been "forcibly removed" from a protest.

A Reuters reporter witnessed the 'red berets', as Armenian special police forces are known, grab one politician by his hands and feet and carry him behind the police cordon.

The public prosecution service confirmed that three opposition leaders had been detained on suspicion of organising an illegal protest.

Almost 200 people have been taken to police stations by law-enforcement officers, the Interfax news agency cited police representatives as saying. Seven people had been taken to hospital, the Ministry of Health said in a statement.

According to the law, the detainees must either be released within 72 hours, or a criminal case can be opened against them.

Opposition leader Mr Pashinyan had earlier held talks with Mr Sarksyan, who walked out of the meeting within minutes after accusing his opponents of trying to "blackmail" the authorities.

"This is not talks, not a dialogue, it's just an ultimatum, blackmail of the state, of the legitimate authorities," Mr Sarksyan had said.

Demonstrators continued to march in large numbers in the capital, blowing vuvuzelas and sounding car horns, despite earlier attempts by police to break up the march with batons and shields.

The police called on protesters to stop the situation from escalating, the Interfax news agency said, quoting an official statement.

"If these demands are not fulfilled, the police has the right to take demonstrators into custody and to use force," Interfax cited the statement as saying.

Tens of thousands of protesters gathered on the capital's central Republic Square in the evening, calling for the continuation of peaceful protests.

In a statement, the EU's foreign policy arm called for more dialogue and a peaceful resolution.

"All those who have been detained while exercising their fundamental right of assembly in accordance with the law must be released immediately," it said.

"It is of utmost importance that all parties involved show restraint and act responsibly."