JERUSALEM, March 26 (Reuters) - Israeli Prime Minister Benjamin Netanyahu on Sunday sacked Defence Minister Yoav Gallant, triggering mass protests, a day after Gallant broke ranks with the government and urged a halt to a highly contested plan to overhaul the judicial system.

As news of the dismissal spread, tens of thousands of protesters, many waving blue and white Israeli flags, took to the streets late at night across the country. Crowds gathered outside Netanyahu's home in Jerusalem, at one point breaching a security cordon.

Some three months since taking office, Netanyahu's nationalist-religious coalition has been plunged into crisis over the bitter divisions exposed by its flagship judicial overhaul plans.

"State security cannot be a card in the political game. Netanyahu crossed a red line tonight," opposition leaders Yair Lapid and Benny Gantz said in a joint statement.

They called on members of Netanyahu's Likud party not to have a hand in "the crushing of national security."

In announcing Gallant's dismissal, Netanyahu's office did not name a replacement nor give any other details. "Prime Minister Benjamin Netanyahu has decided this evening to dismiss Defence Minister Yoav Gallant," it said.

Shortly afterward, Gallant, 64, wrote on Twitter: "The state of Israel’s security has always been and will always be my life's mission."