JERUSALEM (Reuters) - Israeli opposition leader Benny Gantz received an official mandate on Monday to try to form Israel’s next government, and called on Prime Minister Benjamin Netanyahu to join him in a unity administration.

In a sharp blow to Netanyahu, who had declared victory in a March 2 election, 61 of parliament’s 120 legislators voiced support for Gantz, leader of the centrist Blue and White party, in consultations with Israeli President Reuven Rivlin on Sunday.

At a televised ceremony, Rivlin gave Gantz 28 days, with the option of a two-week extension, to assemble a ruling coalition.

But Gantz’s backers include opposing forces - the Joint List of Arab parties, and the far-right Yisrael Beiteinu faction led by former defense minister Avigdor Lieberman - that complicate efforts to form a viable government without wider support.

Netanyahu and Lieberman have proposed a six-month “national emergency government” grouping Blue and White and the prime minister’s right-wing Likud party, to confront the coronavirus crisis.

“I give you my word, I will do all in my ability to establish within a few days as broad and patriotic a government as possible,” Gantz said at the nomination ceremony, without going into details.

Israel has held three inconclusive elections in less than a year, and Netanyahu faces a criminal indictment on corruption charges, which he denies.

Gantz, who in failed coalition negotiations with Netanyahu after a national ballot in September insisted on serving first as prime minister in a “rotating” leadership arrangement, called on his rival to agree to a unity deal now.

“The time has come for an end to empty words,” Gantz said at the ceremony. “It’s time to set aside our swords and unite our tribes and defeat hatred.”