In a first for the court, all 15 justices convened to hear appeals by watchdog groups against a judicial amendment passed by Netanyahu's religious-nationalist coalition in July.
That legislation removed one, but not all, of the tools the court had for voiding government and ministers' decisions if it deemed them "unreasonable".
It could take weeks or even months for the court to rule. But the sense of an unprecedented showdown - pitting judiciary against executive and legislature - gripped Israelis. TV and radio stations aired wall-to-wall coverage of the court session on Tuesday.
The appellants say the amendment removes vital democratic checks and balances, opens the door to corruption and invites abuses of power. They also argue that the swift legislation process was flawed.
The government has said the Supreme Court has no authority to even review amendments to a quasi-constitutional Basic Law.
Justice Minister Yariv Levin, architect of the judicial overhaul, issued a statement deeming the Supreme Court session "a mortal blow to democracy and the standing of the Knesset" by justices he described as both unrepresentative and unelected.
Yair Lapid, centrist head of the parliamentary opposition, said the bill was "warped and thuggish". "It's not worth getting into a national quarrel over such legislation," he said on the social media platform X, formerly known as Twitter.
Netanyahu, who is on trial for corruption charges he denies, says the judicial changes are meant to balance a Supreme Court that has become too interventionist. He has been hazy when asked whether he would abide by a ruling that would quash the new law.
His coalition started its judicial campaign in January, triggering unprecedented protests, spooking investors and sending the shekel down as Western allies voiced concern for the health of Israel's democracy.
With two more appeals scheduled this month, a court ruling could come as late as January, leaving time for the sides to agree on reforms.Â
