WARSAW — When she heard that Poland’s right-wing government was pushing through new laws to give it more control over the country’s courts, Lucyna Relewicz decided the Constitution was in jeopardy and she needed to close her office and take to the streets.

“I came here to defend the Supreme Court,” said Ms. Relewicz, 38, a lawyer from Poznan, in a small cluster of protesters Saturday afternoon. “The courts are the last line of defense of our civil rights and of our democracy and freedom.”

All last week, Poland’s Parliament debated whether to approve a raft of bills, proposed by the governing Law and Justice party, that had come under heavy attack from European Union leaders, human-rights groups and many Poles. By the time the bills were passed early Saturday and sent for President Andrzej Duda’s signature, the peaceful protests that had begun a week earlier had grown and spread to dozens of cities across Poland.

“Thousands of people have been systematically and peacefully demonstrating against the laws and the politicization of the judiciary,” said Kamila Gasiuk-Pihowicz, a member of Parliament for Modern, an opposition party. “Poles suffered too long under Communism. There is no way that Polish citizens or the democratic opposition will allow this to happen to our country.”