YEREVAN -- Opposition activists have blocked several roads in Armenia's capital, Yerevan, to protest plans to elect former President Serzh Sarkisian as prime minister.

The activists also blocked access to a bridge and closed the entrance of a subway station in the city center for some 10 minutes.

Opposition lawmaker Nikola Pashinian, who is leading the rallies, announced earlier that all the roads in downtown Yerevan, the subway system, and several bridges would be blocked. He called on "all citizens" to march to the parliament on April 16.

Police have announced that the rallies are illegal as they violate local residents' right to move freely, but have not interfered in the continuing demonstrations.

Thousands of protesters have been demonstrating in Yerevan since April 13, challenging a major constitutional change approved in December 2015 that weakened the president's powers and strengthened those of the prime minister.

Opposition activists assert that the changes were tailored specially to allow Sarkisian to continue after his two terms as president ended.

Armen Sarkisian -- no relation to Serzh Sarkisian -- was sworn in on April 9. Unlike previous Armenian presidents in the post-Soviet era, he was elected by parliament instead of by popular vote.

The ruling Republican Party has nominated Serzh Sarkisian to the post of prime minister and parliament is expected to vote on his appointment on April 17.