The role of the tribunal, now controlled by PiS-nominated judges, is to rule on the constitutionality of laws passed by the parliament.
The ruling government argues the changes are necessary because the judiciary system is corrupt and benefits the Polish elite.
But as the governing party and its have a comfortable majority, the law was soon passed by parliament in the middle of the night.
The opposition says the changes the ruling party introduces violate the constitution and bring judges under political influence. PiS also argues the judiciary was not purged after the fall of communism, suggesting the ideology affects the branch.
Thousands rallied in Poland’s largest cities on Sunday in protest against the ruling party’s judicial reforms, which the opposition says would kill the judges’ independence and undermine democracy.
PiS refutes such allegations, arguing it is acting on the will of the people, and that the current set of judicial structures were already heavily politicized.
Critics have accused PiS of dismantling the rule of law in Poland and of aiming to stack courts with its own candidates.
PiS supporters argue that the 1989 agreement that led to a gradual – and peaceful – end to communism in Poland didn’t go far enough and in effect shielded ex-communists from prosecution after 1989.
Poland is a relatively new democracy, having overthrown communist rule in 1989 and joined the European Union in 2004.
Members of the National Council of the Judiciary (KRS) will now be chosen by the legislature instead of by an independent board of legal professionals.
The new rules had already been criticized by the Organization for Security and Cooperation in Europe (OSCE) and other global and domestic legal institutions.
The head of the Supreme Court, Malgorzata Gersdorf, who was member of the anti-communist Solidarity movement in the 1980s, has protested the proposed changes as going in the wrong direction and has defended the “highest level of professionalism” of the judges.