Tens of thousands of people took to the streets of Israeli cities to protest against the government of Prime Minister Benjamin Netanyahu.
Demonstrators called for more efforts to negotiate the release of hostages held by the Palestinian Islamist militant group Hamas.
Chants of "elections now" were heard at the protests, with many demonstrators demanding Netanyahu's resignation.
Opposition leader Yair Lapid took part in a rally in the city of Kfar Saba in central Israel.
"They haven't learnt anything, they haven't changed," he said at that rally. "Until we send them home, they won't give this country a chance to move forward."
Israeli media said clashes broke out between protesters and police at a rally in Tel Aviv. Police said one protester was arrested.
Earlier, Israel's army announced that troops had discovered the body of a hostage in the Gaza Strip.
Six months ago, Hamas launched attacks on southern Israel in which almost 1,200 people, mostly civilians, were killed, and 250 people were taken hostage.
In response, Israel launched an offensive in the Gaza Strip in which at least 33,000 people have been killed, according to the Gaza Health Ministry in the Hamas-run territory. The ministry's toll does not differentiate between civilians and combatants.
According to Israeli estimates, 129 hostages are still being held in Gaza, including 34 who are believed to be dead. A November cease-fire deal resulted in the release of 105 hostages.
Before the start of the war between Israel and Hamas in October, Netanyahu's government had already faced mass protests over a controversial judicial reform.
Israel is fighting a 'bloody and difficult war' — Herzog
President Isaac Herzog said that Israel is fighting a "bloody and difficult war" in a statement marking six months since the start of the war between Israel and Hamas on October 7.
"Tomorrow at 6:29 am (0329 GMT), we mark six months since the cruel terror attack and the horrific massacre," Herzog said on Saturday, referring to Hamas's October 7 attack on southern Israel which started the war.
"Half a year since this crime against our sisters and brothers, against our state, this crime against humanity. Six months of a bloody and difficult war," he said.
The office of president is largely ceremonial.
Herzog's statement comes after the army announced that it had recovered the body of a hostage who had been taken into Gaza by Hamas militants on October 7.
Hamas is deemed a terror organization by not only Israel, but also the US, Germany, EU and others.
UK's Sunak: 'This terrible conflict must end'
British Prime Minister Rishi Sunak has called for an end to the war between Israel and the Hamas Islamist militant group.
"We continue to stand by Israel's right to defeat the threat from Hamas terrorists and defend their security. But the whole of the UK is shocked by the bloodshed," he said in a statement.
"This terrible conflict must end. The hostages must be released. The aid — which we have been straining every sinew to deliver by land, air and sea — must be flooded in," he added.
"Today marks six months since the terrorist outrage of 7th October — the most appalling attack in Israel's history, the worst loss of Jewish life since the Second World War," Sunak said.
"Six months later, Israeli wounds are still unhealed. Families still mourn and hostages are still held by Hamas."
Sunak called for an immediate "humanitarian pause" in Gaza followed by a "long-term sustainable cease-fire."
"That is the fastest way to get hostages out and aid in, and to stop the fighting and loss of life," he said.
"For the good of both Israelis and Palestinians — who all deserve to live in peace, dignity and security — that is what we will keep working to achieve," he added.
Later, Foreign Secretary David Cameron stressed that London's support for Israel is not unconditional.
It comes after an Israeli air strike killed seven aid workers, among them three British citizens.
The Israeli Defense Forces said that the strike was a "grave mistake stemming from a serious failure" and dismissed two officers.
Cameron said that there is "no doubt where the blame lies," adding: "This must never happen again."
He urged for there to be a "a wholly independent review to ensure the utmost transparency."
