Around 6,000 gathered in Armenia's capital on Monday in protest at what the opposition calls a power grab by former president Serzh Sarksyan and several people were injured in scuffles with police.
Hundreds of opposition activists have held rallies in recent weeks to protest at Sarksyan's campaign to become prime minister after a constitutional amendment approved in 2015 that transfers governing powers to the premier from the presidency.
The number of protesters has grown to thousands in recent days with protesters blocking the centre of Yerevan, capital of the former Soviet republic.
A Reuters correspondent saw at least three protesters with blood on their hands and legs after scuffles broke out in the city center. Local media quoted a hospital doctor as saying three policemen and one protester had been injured.
Police warned demonstrators they would use tear gas and other means to disperse protesters if they did not leave the Opera House square and unblock the city center.
The ruling Republican party nominated Sarksyan, who served as president for a decade from 2008, as its candidate for prime minister on Saturday.
Sarksyan's ally Armen Sarkissian, a former premier and ambassador to Britain, was sworn in as president last week after being elected by parliament in a vote that was meant to herald the start of a power shift to the premier and parliament.
Under the terms of an amended constitution approved in 2015 by referendum, the presidency will become largely ceremonial.
Parliament, which is dominated by the ruling party, is to decide on Tuesday whether to install Sarkysan as prime minister.
The Dashnaktsutsiun Party, the Republicans' coalition ally, also backs Sarksyan's candidacy, meaning he should easily secure the simple majority needed for election. No other candidates are running.
Opposition leaders have accused Sarksyan of changing the nation's political system to ensure he can remain in power.
Sarksyan had denied any intention of becoming premier until March, when he said his appointment would allow him to share the benefit of his experience as president.
Armenia, a country of three million people in the southern Caucasus, seceded in 1991 from the then-Soviet Union but still relies on Russia for aid and investment. Many Armenians accuse the government of corruption and mismanaging the economy.