The ultra-Orthodox Jews were due to demonstrate in Jerusalem against judgments that have favored civil rights above religious law, including one allowing some shops to open on the Sabbath, the Jewish day of rest.
A simultaneous rally in support of the Supreme Court was scheduled just a few hundred yards (meters) away.
The campaign against the judiciary has split Israel's modern Orthodox Jews. The ultra-Orthodox demonstration has sparked fury among many Israelis who resent the growing power of the fervently-religious Jews, who make up 10 percent of Israeli society.
Prime Minister Benjamin Netanyahu appealed to rally organizers for a postponement, to avoid possible violence, but his appeal was turned down.
Menachem Porush, acting chairman of the Agudat Israel party and organizer of the ultra-Orthodox protest outside the Supreme Court, said the rally would be peaceful.
"We're organizing, in my opinion a million people...who will pray to the blessed holy one to bring the spirit of light into the hearts of the judges so that we can live in Israel according to our beliefs," Porush said.
Poll: Ultra-orthodox have too much power
Most ultra-Orthodox men, recognizable by their long beards and black suits and hats, spend their days in seminars studying religious texts. They get subsidies from the government and are exempt from serving in the Israeli army.
Some West Bank settlers planned to join the ultra-Orthodox demonstration, while others denounced it.
Most modern Orthodox Israelis live ordinary lives, working and serving in the army, while adhering to religious precepts. Israel's secular majority is made up of a relatively small group which rejects all Jewish practice and ritual, and a larger portion that is sympathetic to Judaism, attending synagogue occasionally and celebrating Jewish holidays.
A poll published Friday in the Israeli newspaper Maariv indicated that 75 percent of the people believe the ultra-Orthodox have too much power. Gallup pollsters questioned 560 Israeli adults and quoted a 4.5 percent margin of error.
Though ultra-Orthodox Jews account for just 10 percent of Israel's population, they wield disproportionate power through their political parties in the Knesset, Israel's parliament.
'Operation Static Electricity'
Michal Aharoni, a spokeswoman for the counter-rally, told Reuters: "The protest is not against the ultra-Orthodox, it's in support of the Supreme Court and the system of democracy in Israel."
Thousands of university students are expected to abandon their studies to attend the rally in support along with members of Israel's staunchly secular kibbutz collectives and many religious Jews, Aharoni said.
Yossi Sarid, head of the secular Meretz party, received a death threat from an anonymous caller who demanded the counter-demonstration be cancelled or "the next bullet is for Sarid," Israel radio reported.
About 1,500 police officers will be deployed in "Operation Static Electricity," to keep protesters away from the Supreme Court building and to avoid violence between the rival demonstrators, the radio said.
Israel's Prime Minister Benjamin Netanyahu failed in a last ditch attempt on Saturday night to persuade ultra-Orthodox rabbis, some of them his own coalition partners, to cancel the mass anti-judiciary demonstration.
The ultra-Orthodox pack a powerful political punch as they trade their support of narrow coalition governments for public cash to finance their schools and housing and commitments to uphold public observance of Jewish law.
The rulings in recent months put military exemptions in doubt for up to 30,000 ultra-Orthodox young men and enable the more liberal Reform and Conservative and Reform movements to chip away at the Orthodox monopoly over Judaism in Israel.