Nils Muiznieks, the human rights commissioner for the Council of Europe, said the Polish government had neglected "compelling contrary advice" before voting to give parliament, rather than judges, the power to choose members of the National Council of the Judiciary. Law and Justice won the 2015 elections and controls the parliament.
The Polish government has said changes are needed to reform an inefficient and sometimes corrupt judicial system.
Legislation that was adopted last week has drawn condemnation from European Union politicians and from Poland's opposition.
Another draft law requires that all current Supreme Court judges retire and be replaced with new appointees selected by the justice minister, and the work of the court reorganized.
The Supreme Court building stands in Krasinski Square, site of the Warsaw Uprising monument and where US President Donald Trump said in early July Poland and the United States shared a "commitment to safeguarding the values (of) freedom, sovereignty and the rule of law".
Prince George & Princess Charlotte Arrive in Poland in Adorable Style
Later this evening the duke and duchess are scheduled to attend a formal dinner at the Royal Łazienki park. WireImage Prince William bending down to speak to his son at Warsaw airport.
The opposition says the changes the ruling party introduces violate the constitution and bring judges under political influence.
The party has also been adamant in its promotion of a far more muscular and executive-driven democracy, which critics argue undermines the separation of powers and the rule of law.
The new rules had already been criticized by the Organization for Security and Cooperation in Europe (OSCE) and other worldwide and domestic legal institutions.
Poland's most powerful politician insisted Friday that "radical changes" are necessary to heal the nation's judiciary and vowed to push ahead despite vehement protests from Poland's opposition and European bodies.
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.