Calling for calm, in a special televised address Duda, who hails from Poland's ruling conservative Law and Justice (PiS) party, said he would veto bills to reform the Supreme Court and a powerful judges' ethics council.
Voicing concern over social unrest, Duda said: "I do not want this situation to deepen, because it deepens division in society.” He admitted that he could face criticism for his veto decision.
Thousands have taken to the streets across Poland over the last week to protest at the PiS government’s planned changes to the judicial system and calling for the president to veto them.
Duda said that “wise” changes to the judiciary were needed, but added that he was against strengthening the influence of the Attorney General over the Supreme Court.
Duda has up to now signed into law most bills passed by the PiS-dominated parliament.
The lower house needs a three-fifths majority to overturn a presidential veto. Law and Justice does not command a majority of that size.
'Inefficient' judicial system
PiS has said sweeping changes are needed to reform an inefficient and sometimes corrupt judicial system, accusing judges of being an elite, self-serving clique often out of touch with the problems of ordinary citizens.
In his address, Duda announced on Monday that within two months he would draw up new bills on the Supreme Court and the influential National Council of the Judiciary (KRS), a body tasked with safeguarding the independence of courts and judges.
The opposition warned that changes to the Supreme Court, under a bill approved by the Senate following a stormy debate in both houses of parliament, undermined judicial independence and accused Poland’s ruling conservatives of trying to stack courts with its own candidates.
The European Commission last Wednesday said it was ready to take action against Poland for violating EU laws, warning that the controversial overhaul of the country's judicial system threatens the independence of courts.
Prime Minister Beata Szydło then said the Polish government would “not give in to pressure” at home and abroad to stop changes to the judicial system.
In an immediate reaction to Duda’s announcement on Monday, Jacek Sasin, a PiS MP, said: “I admit I’m surprised by this decision by the president.”
He added Duda's move "means that we will have to wait a long time to reform the Polish judiciary."
Deputy Prime Minister and Finance Minister Mateusz Morawiecki said he was “disappointed” by the president’s decision.
But senior Catholic churchman Archbishop Stanisław Gądecki, head of the Polish Episcopate Conference, thanked Duda for his decision, saying in a letter to the president that "authentic democracy is possible only in a state governed by the rule of law”.
A Supreme Court reform bill, passed by both houses of Polish parliament last week, would have forced the court's existing justices into retirement, while giving the president powers to choose who to reinstate.
A KRS bill would have seen the terms of 15 of its members who are judges phased out and their successors selected by parliament -- rather than by other judges as up to now.
A third bill -- which a spokesman for Duda said on Monday the president would sign into law --changes the way the heads of district and appeals courts are appointed and dismissed, giving more power to the justice minister, and making the allocation of cases to judges random.
Grzegorz Schetyna, head of the Civic Platform (PO), Poland’s largest opposition party, said Duda should also veto the third bill.