The protesters were encouraged to act by Armenian MP Nikol Pashinyan, leader of the opposition Civil Contract party.
Serzh Sargsyan, who served as president from 2008 until earlier this year when he stepped down because of term limits, is set to be approved as prime minister on Tuesday.
While presidential votes have typically been contentious affairs in Armenia, Sarkissian's election was initially met with comparative shrugs, and not just because the real power will now shift to the prime minister's office, which Sargsyan is widely expected to slide into. Several hundred people sat or laid down on pavements, blocking roads leading to the parliament building and universities. Police urged protesters not to violate public order.
Some built barricades using cast-iron benches and metal trash cans.
Authorities called on him to end the protests, which they threatened to break up by force.
Forty-six citizens, including six police officers, applied for medical assistance as of 16:30, the Ministry of Health reported.
Opposition politicians say the shift to a parliamentary republic with a powerful prime minister has been created to increase Serzh Sarkisian's grip on power in the impoverished Moscow-allied country.
The new system sees the president's powers weakened and the prime minister taking a dominant role.
Before Pashinyan said about the transition to action, "locking" of transport and the Parliament of Armenia.