Malgorzata Gersdorf has described the new laws as a 'purge' of Poland's highest court
Poland's top judge has shown up for work, defying a retirement law that has led to protests in the capital Warsaw.
The rule is one of several controversial laws introduced by the governing right-wing Law and Justice Party (PiS).
Under the judicial reforms - which came into force yesterday - 27 out of 72 Supreme Court judges in Poland are being forced to retire, with the retirement age being lowered to 65.
Judges can be granted an extension if approved by the country's president - currently an ally of the PiS - but almost a dozen judges have reportedly refused to apply in protest against the new law.
Critics of the retirement law and other judicial reforms have hit out the laws, with human rights groups warning the independence of the judiciary is at risk by giving politicians more control over appointments.
The EU has also sharply criticised the legislation, and has launched legal action against Poland - with the European Commission saying it believes the measures are in breach of one of the EU's common values by "undermining the principle of judicial independence".
First President of the Supreme Court Malgorzata Gersdorf entered the court as normal this morning, in defiance of the new law.
The chief justice has described the rules as a 'purge' in the guise of reform.
Speaking to reporters as she entered the building, Mrs Gersdorf said: "My presence here is not about politics: I am here to protect the rule of law."
Mrs Gersdorf is due to serve in her role until 2020, and insists her position is constitutionally protected.
There have been growing protests in Poland over the new laws, with hundreds gathering outside the Supreme Court this morning in support of Gersdorf and the other judges opposing the laws.
Addressing MEPs in Strasbourg this morning, Polish Prime Minister Mateusz Morawiecki defended his government's reforms.
In comments quoted by the Financial Times, he argued: "Every country has a right to set up its own legal values with its own traditions".
He also urged member states to listen to dissatisfied voters:
Prime Minister @MorawieckiM in #Strasbourg: You can call it populism, but, sooner or later, the following question must be asked: is meeting the expectations of our citizens truly populistic or maybe – it is the essence of democracy.— Chancellery of the Prime Minister of Poland (@PremierRP_en) July 4, 2018