The plans, which the government says are needed to curb overreach by activist judges, have drawn fierce opposition from groups including lawyers, and raised concerns among business leaders, widening already deep political divisions in Israeli society.
"They want to turn us into a dictatorship, they want to destroy democracy," the head of the Israeli Bar Association, Avi Chimi said. "They want to destroy judicial authority; there is no democratic country without a judicial authority."
Netanyahu has dismissed the protests, now in their third week, as a refusal by leftist opponents to accept the results of last November's election, which produced one of the most right-wing governments in Israel's history.
The protesters say the future of Israeli democracy is at stake if the government succeeds in pushing through the plans, which would tighten political control over judicial appointments and limit the Supreme Court's powers to overturn government decisions or Knesset laws.
As well as threatening the independence of judges and weakening oversight of the government and parliament, they say the plans will undermine the rights of minorities and open the door to more corruption.
"We are fighting for democracy," said Amnon Miller, 64, among crowds of protesters, many bearing white and blue Israeli flags. "We fought in this country in the army for 30 years for our freedom and we won't let this government take our freedom."
Saturday's protests, which Israeli media said were expected to draw more than 100,000 people to central Tel Aviv, come days after the Supreme Court ordered Netanyahu to fire Interior Minister Aryeh Deri, who leads the religious Shas party, over a recent tax conviction.
The new government, which took office this month, is an alliance between Netanyahu's Likud party and a clutch of smaller religious and hard-right nationalist parties which say they have a mandate for sweeping change.
Netanyahu, who is himself on trial on corruption charges which he denies, has defended the judicial reform plans, which are currently being examined by a parliamentary committee, saying they will restore a proper balance between the three branches of government.
Likud politicians have long accused the Supreme Court of being dominated by leftist judges who they say encroach on areas outside their authority for political reasons. The court's defenders say it plays a vital role in holding the government to account in a country that has no formal constitution.
A survey released by the Israel Democracy Institute last week showed trust in the Supreme Court was markedly higher among left-wing Israelis than among those on the right, but that there was no overall support for weakening the court's powers.
