Opponents of Israeli PM Benjamin Netanyahu say 100,000 people have rallied against the government and to demand a Gaza hostage deal.
The rallies in Tel Aviv and other cities came after the Israeli Defense Forces (IDF) recovered the body of hostage Elad Katzir.
Protesters chanted "elections now", and "Elad, we're sorry", local media said.
Both Israelis and Palestinians are preparing to mark six months of war on Sunday.
Anti-government protesters were joined by families of hostages held in Gaza. Demonstrators expressed their frustration with the government's inability to free the around 130 hostages who remain in Gaza, held by Hamas and its allies.
Earlier on Saturday the IDF recovered the body of Elad Katzir, who was seized and taken to Gaza during the Hamas attacks on southern Israel on 7 October. He appeared alive in a hostage video released in January.
"Elad Katzir managed to survive three months in captivity. He should have been with us today. He could have been with us today," protester Noam Peri told a BBC reporter.
Organisers said the protesters had rallied in around 50 locations across Israel. These were the latest of a series of huge anti-government protests demanding that Prime Minister Netanyahu step down, amid fury that he has failed to free the remaining hostages.
A car ploughed into crowds at the protest in Tel Aviv, injuring five. The cause of the incident was unclear.
On Sunday - exactly six months since the Hamas attacks triggered the war in Gaza - negotiators plan to meet in Cairo to attempt to reach a ceasefire in the brutal war.
According to some media reports, CIA Director Bill Burns and Qatari Prime Minister Mohammed bin Abdulrahman Al Thani will join negotiators from Egypt, Israel and Hamas.
Katzir's sister, Carmit Palty Katzir, blamed Israeli authorities for her brother's death in a post on social media, saying he would have returned alive had they agreed a new truce deal.
"Our leadership is cowardly and driven by political consideration, which is why this deal has not happened yet," she wrote on Facebook.
"Prime Minister, war Cabinet, and coalition members: Look at yourself in the mirror and say if your hands didn't spill blood."
The 7 October Hamas attacks killed about 1,200 people, most of them civilians.
Over 33,000 people have been killed during Israel's offensive in Gaza since then, the Hamas-run health ministry says - many of them women and children.
According to Israeli counts, 253 Israelis and foreigners were taken during the Hamas attacks.
About 129 hostages remain unaccounted for after being kidnapped - at least 34 of them are presumed dead. 12 bodies have been recovered by the IDF.
Israel gives a slightly higher official figure because it includes four people taken hostage in 2014 and 2015. Two of these are believed to have died.
- This story was first published by BBC News