Israeli opposition head Tzipi Livni set a leadership vote for her centrist Kadima party for next month on Wednesday, citing the prospect of an early general election.

Livni, a former foreign minister, said Kadima's so-called primary would be held on March 27. She said she was confident of re-election, but her main rival, former defence chief Shaul Mofaz, has insisted he could unseat her.

Livni, 53, said Kadima's main goal was toppling what she called Prime Minister Benjamin Netanyahu's extremist government.

Israeli elections are no longer far on the horizon, and are beginning to grow closer, and Kadima must and shall be ready for Israeli elections, Livni told party faithful at their headquarters near Tel Aviv. She had earlier been quoted as saying there was no need for a leadership contest.

Netanyahu has triggered speculation that the election, legally due in 2013, may be moved up, by scheduling a leadership contest in his right-wing Likud party for January 31.

Livni said she saw herself as the only alternative to Netanyahu, who she blames for limited progress achieved in peace talks with the Palestinians since he rose to power three years ago.

Kadima has also pummelled Netanyahu's handling of social divisions between religiously Orthodox and Israel's majority secular Jews, and high housing and food costs blamed for unprecedented social protests in Tel Aviv last year.

Yet these issues take a back seat to Israel's focus on security threats, with a wary eye on rising Islamist influence in neighbouring Arab revolts and what it sees as an existential risk posed by Iran's nuclear programme.

Opinion polls predict Netanyahu's right-wing Likud party, which outpolled Kadima in the 2009 parliamentary election, would likely win again, if an early election were held.

The left-of-centre Labour party headed by Shelly Yechimovitch, now one of Parliament's smaller parties, may give Kadima a keen contest for second place, riding on its outspoken backing for the recent social protests, some surveys show.

With no serious contenders for his party leadership job, Netanyahu's bid is seen by analysts as a bid to use his current popularity as a springboard for a wider mandate against far-right and religious rivals inside his squabbling coalition.

Under Israeli law, elections are held every four years, but are often held earlier than that. Pundits see a possibility the next poll may be moved up to later this year.

Kadima, the once dominant party founded by former prime minister Ariel Sharon, got more votes than Likud in the 2009 election but Livni failed to muster enough backing from religious and other parties to form a coalition, handing Netanyahu the victory.

Livni's rival Mofaz, speaking at a separate news conference, said she was finished as Kadima leader.

Having only narrowly defeated Mofaz in a 2008 primary, their contest seemed too close for now to call. Both are former Likud members, and their differences are seen more as personal than over policy. Other candidates may also join the race.

(Writing by Allyn Fisher-Ilan Editing by Maria Golovnina)