The head of Poland's Supreme Court arrived for work surrounded by hundreds of supporters, rejecting a controversial law forcing dozens of senior judges to retire early.
Chief Justice Malgorzata Gersdorf, 65, had been told to step down at midnight on Tuesday and a replacement was named.
She has branded the reforms, which require judges to retire at 65 instead of 70, a "purge".
Poland's prime minister defended his government's drive to enforce changes.
"Every EU country has the right to develop its judicial system according to its own traditions," Mateusz Morawiecki said in a speech to the European Parliament on Wednesday.
On Monday the European Union launched legal action against Poland's right-wing government, saying the law undermined judicial independence. Some Euro MPs listened to the Polish leader's speech behind signs that read "rule of law".