"Courts are remnants of the former corrupt authorities. People have no trust in the judiciary and therefore it lacks legitimacy," Pashinyan told a televised cabinet meeting.
"The time has come for a surgical intervention" with all judges subjected to a thorough vetting during a "transitional" period, he said, Presstv Reported.
He said judges who "believe they cannot be just" and those whose rulings were found unfair by the European Court of Human Rights should resign.
"If necessary, we will adopt constitutional amendments. All this will be done in conformity with international law," he said, asking for support from Western partners.
Pashinyan's appeal for mass demonstrations followed a court ruling to release from pre-trial detention former president Robert Kocharyan charged with staging a "coup" a decade ago.
The symbolic protest came as Pashinyan tries to consolidate power in the ex-Soviet country a year after leading a popular revolt against former president Serzh Sarkisian and corrupt elites.
'Second stage of the revolution'
Pashinyan said he "counts on the assistance of Armenia's international partners" in implementing the judiciary reform which he dubbed the "second and most important stage of the revolution."
Braving torrential rain, dozens of Pashinyan supporters set up pickets outside the Constitutional Court, as well as Yerevan's City Court building early Monday, an AFP correspondent reported.
Later in the afternoon, Pashinyan called for an end to the blockade, writing on his Facebook page, that "the demonstration has achieved its goal."
Kocharyan is accused of tipping a 2008 presidential ballot in favor of his hand-picked ally Sarkisian. He faces up to 15 years in prison if found guilty of "overthrowing the constitutional order."
He was arrested in July last year, then briefly released but re-arrested again in December and has since then remained in pre-trial detention.
On Saturday, a Yerevan court released him from custody, pending a final decision in the case.
Pashinyan denounced the ruling -- which has sparked widespread outrage in the impoverished Caucasus country -- as politically motivated.
Kocharyan led Armenia for a decade up to 2008 when Sarkisian was elected and remained in charge until the 2018 revolt against an attempt to extend his power forced him to resign.
He has rejected the charges brought against him as "political vendetta."
After the 2008 election, tensions erupted into violent clashes between riot police and supporters of the defeated opposition candidate, who denounced the vote as fraudulent.
Eight protesters and two officers were killed.