The 63-year-old ended his second and final presidential term last week and has been nominated by the ruling party for the post of prime minister.
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. Authorities said 46 people - including six police - sought medical help. Later on, the most serious incident of the demonstration so far happened when the passengers of one of the vehicles attacked the protesters as the vehicle drove through the barrier.
On the same day, protesters led by Pashinyan seized the building of Armenia's Public Radio for around an hour.
There were media reports that Tuesday's NA special session to elect the PM would not be convened in the parliament building. Some built barricades using cast-iron benches and metal trash cans.
The protests were announced by the opposition Yelk (way out) coalition on 12 April, a day after Serzh Sargsyan's announcement, while lighting coloured smoke flares on the assembly floor of the Armenian parliament.
Armen Sarkisian took over as president from Sargsyan on Monday under a new system in which the president's powers will be weaker and the prime minister will dominate. The Constitution of Armenia was changed, transforming it from presidential to a parliamentary state.