JERUSALEM, June 6 (Reuters) - Prime Minister Benjamin Netanyahu said on Sunday (Jun 6) a newly formed Israeli coalition that is poised to unseat him was the result of "the greatest election fraud" in the history of democracy.

He made his sweeping accusation at a time when Israel's domestic security chief has warned publicly about the prospect of political violence. Netanyahu focused his allegations on a broken campaign promise from the man set to replace him as prime minister, nationalist Naftali Bennett.

Bennett had pledged not to partner with left-wing, centrist and Arab parties, but on Wednesday announced with opposition leader Yair Lapid that they had formed a governing coalition with factions from across the political spectrum.

Under a rotation deal, Bennett will serve first as prime minister, followed by Lapid.

No date has been set for a vote in parliament to approve the new government, which follows an inconclusive Mar 23 election, but it is widely expected to be sworn in on Jun 14.

"We are witnessing the greatest election fraud in the history of the country, in my opinion in the history of any democracy," Netanyahu said in comments to legislators of his right-wing Likud party.

"That's why people justifiably feel deceived and they are responding, they must not be shut up," he said in the remarks, which were broadcast live and referred indirectly to Bennett's campaign promise not to team up with Lapid and others.

Netanyahu, Israel's longest-serving leader, has been in office since 2009, and his tenure has been clouded by an ongoing corruption trial, in which he has denied any wrongdoing.

The prospective new government caps political jockeying since the election - Israel's fourth in two years. People angry at the alliance have held protests outside the homes of opposition politicians, whose security has been beefed up after threats on social media.