Pashinian was detained during a demonstration in the Armenian capital, shortly after failed talks between him and Prime Minister Serzh Sarkisian.
Pashinian was confronted by masked police officers and other security personnel as he led a crowd of several hundred supporters marching to the city's southern Erebuni district. The police fired stun grenades to stop the march.
Police later tried to disperse the crowds gathered in various parts of Yerevan, and there have been reported clashes between police officers and some of the protesters.
According to Police, 232 people have been detained in connection with the current anti-government protests.
The Health Ministry said seven people have been taken to hospital in relation with the unrest.
More protesters were detained in the evening on the central Republic Square.
The Interior Ministry said in a statement that it made the decision to "disperse demonstrators," adding that police were "entitled to carry out arrests and use force."
The Prosecutor-General's Office earlier said that Pashinian and two other detained opposition lawmakers, Sasun Mikaelian and Ararat Mirzoian, will be held for 72 hours.
It said in a statement that the three organized unsanctioned rallies and urged supporters to block streets and entrances to state buildings. It claimed that participants of those gatherings assaulted police officers.
The statement did not say whether there will be criminal charges against the three lawmakers, who are protected by parliamentary immunity. Their prosecution would require parliament to remove their parliamentary immunity.
Meanwhile, thousands of Pashinian's supporters marched in the capital , Yerevan, chanting his name. The march was led by four priests in clerical outfits.
Earlier, Sarkisian walked out a meeting with Pashinian after accusing the opposition of "blackmail."
Protesters also rallied in Armenia's second-largest city, Gyumri and in Vanadzor, the third-largest city in the country of about 3 million people. Several protesters were detained in Vanadzor on April 21.
