Jerusalem (AFP) - Three days ahead of Israel's general election, there was growing uncertainty Saturday over who will win the premiership, with polls showing incumbent Benjamin Netanyahu trailing his centre-left rivals.

The final two surveys released Friday night by private television channels gave the Zionist Union, headed by Labour leader Isaac Herzog, a four-seat lead over Netanyahu's Likud.

A poll by Channel 10 showed Likud winning 20 seats, compared with 24 for the Zionist Union, while a survey issued by Channel 2 also showed Likud four seats behind, 22 to 26.

The results echoed surveys released earlier Friday -- the final day that opinion polls could legally be published before Tuesday's election -- which both predicted a win for the Zionist Union.

But Israel's complex electoral system, where many parties are vying for power, means the task of forming a new government does not automatically fall to the winning candidate or list.

Israel's new premier will be the one who can build a coalition commanding a majority of at least 61 seats in the 120-strong Knesset.

That task will be all the harder as there are at least 11 party lists to reckon with from across the political spectrum as well as ultra-Orthodox and Arab parties.

Under the proportional system, voters choose party lists rather than individual candidates, with seats distributed according to the percentage of the vote received.

Analysts believe the next three days will be crucial, as 20 percent of voters have said they are undecided.

But it could take weeks of negotiations before the name of the new prime minister is known.

Friday's polls had put The Joint List, a newly formed alliance of Israel's main Arab parties, in third place, with 13 seats and predicted that the centre-right Yesh Atid could win 12 seats.

Although consistently trailing in the polls, Netanyahu has come out fighting, and analysts say he may be better placed than Herzog to form a coalition.

- Right wing rally on Sunday -

On Channel 2's "Meet the Press" programme Saturday night, the two came face-to-face -- albeit with Netanyahu appearing by video link.

Their brief exchange focused on security and diplomatic issues.

Campaigning is due to close on Sunday night with a major rally in Tel Aviv by right-wing parties -- a week after the centre-left mobilised thousands of supporters in the coastal city.

It is not clear if Netanyahu, who is trying to clinch a third consecutive term in office, will attend Sunday's rally.

But the feisty prime minister has stepped up public appearances in recent days, giving interviews to major newspapers and television channels to push the centrepiece of his campaign: security.

Netanyahu has warned that a win by the Zionist Union -- a coalition of Herzog's Labour party and centrist HaTnuah of former chief peace negotiator with the Palestinians Tzipi Livni -- would affect security.

On Thursday he told the Jerusalem Post daily that a Zionist Union victory "will cause such a monumental shift in policy that it is a danger, and anyone who wants to stop it has to vote Likud to narrow the gap."

The premier warned such a result would mean Israel having to share Jerusalem with the Palestinians, who want to set up the capital of their future state in the mostly Arab eastern part of city Israel considers it eternal capital.

"I am not dealing with retirement. I am dealing with victory," Netanyahu said.

The Zionist Union has said if it wins Herzog and Livni would share the premiership, each serving two years.

Herzog has received endorsement by several prominent figures in Israel, including former president Shimon Peres, who said Thursday he was "level-headed".

On Friday, Yuval Diskin, a former Shin Bet internal security agency chief, wrote on Facebook that Herzog should become the next premier "because Netanyahu has failed in almost every area".

Herzog told Channel 2 on Saturday the election was "a choice between despair and hope".

"The Israeli public is fed up with Netanyahu and knows that I'm the only one who can replace him," he said.