Demonstrators blocked numerous roads across Israel, including a major highway in the coastal city of Tel Aviv.
They waved blue-and-white Israeli flags as well as yellow flags symbolising solidarity with the hostages.
Demonstrators called for an end to the war in the Gaza Strip and the immediate release of the Hamas hostages.
The protests were criticised by Prime Minister Benjamin Netanyahu and ultranationalist Finance Minister Bezalel Smotrich.
"Those who are calling for an end to the war today without defeating Hamas are not only hardening Hamas' stance and pushing off the release of our hostages, they are also ensuring that the horrors of the October 7 will recur again and again and that our sons and daughters will need to fight again and again in an endless war," Netanyahu told a cabinet meeting.
Smotrich described the protests in a post on X as a "bad and harmful campaign that plays into Hamas' hands".
The protesters urged the Israeli government to reverse its decision to capture Gaza City and other areas of the Gaza Strip, as they believe this will endanger the 20 hostages believed to still be alive.
More than 30 people were arrested, according to police.
Israeli President Isaac Herzog visited Hostage Square in central Tel Aviv and called on international decision makers to pressure Hamas to release the hostages.
"I want to tell the world: stop the hypocrisy!" he said.
In Jerusalem, water cannons were used against protesters.
The organisation representing the families of the hostages had called for a country-wide strike on Sunday, the start of Israel's working week.
Israel was moving closer to intensifying its war against Hamas in Gaza City, military chief Eyal Zamir said during a visit to troops in the Gaza Strip on Sunday.
The plan for an expanded offensive, which was approved Israel's security cabinet earlier this month, foresees the capture of Gaza City and central refugee camps in order to dismantle remaining Hamas strongholds in the war-shattered Palestinian territory.
The aim is to possibly take control of the entire Gaza Strip, and could require relocating roughly a million Palestinians currently in Gaza City to other parts of the territory.
"Soon we will move on to the next phase of Operation Gideon's Chariots, in which we will continue to enhance the strikes against Hamas in Gaza City until its decisive defeat," the chief of staff of the Israeli Defence Forces (ISF) said, using Israel's codename for a ground offensive launched in early May.
"The IDF will deploy all its capabilities, on land, in the air and at sea, in order to strike Hamas decisively," Zamir said in an IDF post on Telegram.
Zamir, who the IDF said made the comments during a field tour on Sunday, said the military had already achieved many of its objectives in earlier operations: "Hamas no longer possesses the same capabilities it had before the operation; we dealt a severe blow."
He added that the military now "bears the moral duty to bring the hostages home, both alive and fallen".
During the Islamist Hamas-led massacres on October 7, 2023, militants seized about 250 hostages.
Of the roughly 50 hostages still in the Gaza Strip, about 20 are believed to be alive.
