The protesters, protesting against the appointment of ex-President of Armenia Serzh Sargsyan Prime Minister, was blocked by several Central streets of Yerevan.
The RPA will nominate Serzh Sargsyan as candidate for the next Prime Minister.
The policemen were pushing the protesters to the sidewalks, several people were taken to custody.
Meanwhile, the national police issued a written appeal to Pashinian saying that the protests will be forcibly broken up if he continues defy their warnings.
Sarkisian had promised in 2014 that he would "not aspire" to be prime minister if Armenia switched from a presidential to a parliamentary system as a result of the referendum. We call on the organizers of the meeting and some of its participants to refrain from violating the rule of law, not to jeopardize the rights and freedoms of fellow citizens, legitimate interests of organizations.
"My Step" initiative leader, opposition's Civil Contract Party member and National Assembly (NA) "Way Out" (Yelk) Faction head Nikol Pashinyan jumped over the barbed wire fence set up by police forces, while protesters followed his lead.
Sarkisian, 63, ended his second and final presidential term last week.
On Tuesday, the Armenian parliament will vote on the nation's new prime minister.
Earlier on Monday, demonstrators marched through the centre of Yerevan and blocked streets after Pashinian called on Armenians to prevent Sarkisian's political transition.
"In Armenia there is something unprecedented: the same person wants to become the leader of the country for the third time".
"No one will prevent us from voting tomorrow for our candidate Serzh Sarkisian", he told AFP. The rallies began on Friday when more than 4,000 people took part.
The developments raised tensions ahead of the parliament session that government critics say will cap a "power grab" by Sarkisian, enabling him to retain control despite leaving the presidency less than two weeks ago.
Police have detained about thirty people.