Six of them are police officers, the Armenian Health Ministry press service said.
The source did not provide further details on the number of the detained opposition activists and the reasons behind their detention, claiming they obtain no such information.
Serzh Sargsyan, who served as Armenia's 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. Sargsyan received 77- for and 17- against votes.
Moments after a deputy chief of Yerevan's police department, Valeri Osipian, warned Pashinian against "provocative", actions the crowd pushed through the first lines of riot police. A police officer underwent a surgery, as his vein was cut.
"As of a political figure, he has always stood out for his disobedient views and honest criticism of the authorities, for which I have always liked him", 32-year-old Armen Ohanian told RFE/RL as he protested to keep "this vicious system from being reproduced".
Several thousand people in Yerevan protested against the nomination of former president Serzh Sarkisian as prime minister under a new parliamentary political system.
Demonstrators have vowed to keep protesting round the clock.
The pro-Russia politician ended his second and final presidential term last week, but the ruling party has nominated him for the post of prime minister.
Anti-government protesters in Armenia have been urged to continue their fight.
"Serzh Sarkisian will not be Armenia's prime minister", declared the 42-year-old former journalist.
"Pashinian has used his parliamentary mandate for the interests of citizens", says Artur Sakunts, head of the Helsinki Citizens' Assembly Vanadzor Office.
"About three dozens of our supporters are now in the police departments, I call on their friends and relatives not to go to the police, but to block the streets and avenues of the capital city", Pashinyan said.
"We can not let this happen".
The protesters marched towards the parliament building after managing to shut down traffic in most of the city center. For the first time, Armenia's president was selected by the National Assembly, rather than by popular vote.
Pashinian said the closure of the streets leading to the parliament should start already before midnight.