JERUSALEM — The Israeli Supreme Court on Tuesday was hearing a case against a contentious law that defines the country as the nation-state of the Jewish people, marking the beginning of a landmark challenge against legislation that opponents say discriminates against minorities.

Critics say the law further downgrades the status of Israel’s Arab minority, which makes up around 20 percent of the country’s population. Proponents claim the legislation merely enshrined Israel’s existing Jewish character into law.

The 15 petitions filed by Arab rights groups and other civil society organizations seek to have the country’s Supreme Court strike down the law. The petitions pose a major challenge to the 2018 law and are being heard by a panel of 11 justices, the court’s largest possible configuration.

“This is the first time in Israeli legal history that the Supreme Court has to deal with the legal status of the Palestinian minority in Israel,” said Hassan Jabareen, founder of Adalah, an Arab minority rights group petitioning the court, ahead of the hearing.

The nation-state law was approved by the Knesset in July 2018. It defines Israel as the “nation-state of the Jewish people” and adds that “fulfilling the right to national self-determination in the state of Israel is unique to the Jewish people.” It also downgraded Arabic from an official state language to one with “special status.”

The law’s passage prompted vocal opposition by the country’s Arab minority, particularly among Druze Israelis, who serve in the military and saw themselves demoted to second class citizens.

As the judges heard arguments against the law from petitioners, Israeli Prime Minister Benjamin Netanyahu wrote on Facebook that the Supreme Court “has no authority to debate the validity of Basic Laws,” adding that the court “is not an absolute ruler” and called for legislation to limit judicial authority.