"We are on our way to the office of the prosecutor general, we'll ring the building and demand that the prosecutor general arrest Pashinyan", Arshak Zakaryan, one of the protest organisers, said.
At one point, the protesters pushed forward, and the police stationed next to the building had to push back; then detentions began.
Yerevan has been seeing protests regularly since 10 November when Azerbaijani President Ilham Aliyev and Armenian Prime Minister Nikol Pashinyan signed an agreement on a ceasefire in Nagorno-Karabakh that was brokered by Russian President Vladimir Putin.
The deal ended the armed conflict which reignited in late September. Under the agreement, Baku regained control of the territories occupied by Armenians before the outbreak of hostilities, prompting a public outcry in Armenia.