Israel's Labour party leader has joined forces with the former justice minister in a bid to unseat Prime Minister Benjamin Netanyahu in the country's snap election in March.

Opposition Labour party leader Isaac Hertzog and centrist Tzipi Livni of the HaTnuah party on Wednesday announced their alliance at a televised news conference featuring the slogan "Together We Win".

"When we receive the mandate... I shall serve as prime minister for the first two years and Tzipi Livni will serve as prime minister for the second half" of the four-year term, Hertzog said.

The extremists... are turning our country into an isolated state, closed and alienated from its own citizens.

Tzipi Livni, former justice minister

"Today the Zionist centre has risen against the parties of the extreme right," said Livni, formerly Israel's chief negotiator with the Palestinians, who was fired from government last week by Netanyahu, who accused her of working against his right-leaning coalition from within.

The joint list of candidates is expected to run on a platform emphasising the peace process with the Palestinians and greater economic equality.

Opinion polls give the Herzog-Livni alliance about 24 seats in the 120-seat parliament which is slightly more than their current total.

That is two or three more seats than polls give Netanyahu's free-market, settler-friendly Likud. 

Likud 'extremists'

Livni began her political career in Likud before following the late Ariel Sharon when he founded the Kadima party, then setting up her own HaTnuah before the last election.

She said that today's Likud has been taken over by the far-right.

"The extremists... are turning our country into an isolated state, closed and alienated from its own citizens."

Likud said the new alliance, if it took power, would make sweeping concessions in talks with Palestinian leader Mahmoud Abbas.

"There is no doubt what he could expect from such a radical left-wing government; dangerous concessions in Jerusalem and throughout our homeland," Likud said in a statement.

Netanyahu fired Livni and Finance Minister Yair Lapid, head of the centrist Yesh Atid party, before calling an early election, which parliament set for March 17.

Livni said that HaTnuah and Labour would field a joint list of candidates and that she expected unnamed other parties to enter the alliance.

The same tactic was used in the 2013 election when Likud and Foreign Minister Avigdor Lieberman's Israel Beitenu ran on a joint ticket, taking 31 seats.

Source: Agencies