Footage posted online shows crowds gathering on a motorway in the Israeli city of Tel Aviv, blocking traffic and lighting fires.
There have also been similar demonstrations in Jerusalem, Haifa, Caesarea, and other locations, according to The Times of Israel.
There have been regular rumours Mr Gallant was on the way out but refused to go.
He had argued for a hostage deal in Gaza and clashed with other parties in Israel’s coalition over drafting members of the ultra-Orthodox Jewish community into the military.
In a televised statement after he was sacked, he said Israel was navigating through the fog of battle and “moral darkness”. He also called for an inquiry into the failures of 7 October 2023.
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In contrast, Mr Netanyahu described “significant gaps” and a “crisis of trust” for his decision to replace Yoav Gallant with a longtime loyalist.
Like the prime minister, Mr Gallant’s career was deeply scarred by the Hamas-led attacks originating from Gaza, in which gunmen killed about 1,200 people and seized more than 250 hostages.
He knew he was living on borrowed time – Mr Netanyahu’s first attempt to sack him last year failed after some of the biggest protests ever seen in Israel.
The prime minister backed down that time, but relations between the pair have never recovered and they wrangled constantly as the war in Gaza passed its first anniversary.