Greece's election hung on a knife-edge Saturday as radical former premier Alexis Tsipras retook a slight lead in the final hours of a tight race against the conservatives for the helm of government.

Hours before a midnight ban on voter surveys expired Friday, a brace of polls forecast victory for Tsipras over conservative party chief Vangelis Meimarakis by margins ranging from 0.7 to 3.0 percentage points.

However, pollsters advise caution with many recalling the 2000 election that was decided by a mere 72,000 votes.

A victory for Syriza would deliver "a key message for Europe", Tsipras told his closing rally in Athens on Friday, referring to the refugee crisis and EU economic woes.

"Do we want a Europe of austerity or one of solidarity and democracy?" he said.

Wearing a white shirt with the sleeves rolled up, Tsipras said voters would say "no to this old system of corruption, no to the enshrining of the oligarch establishment."

At Tsipras's side were some of Europe's new radical leaders, including Pablo Iglesias, who heads Spain's Podemos party.

With nine parties trying to enter parliament, whoever wins is unlikely to secure an outright majority and Tsipras's Syriza party could well need to ally itself with one of the parties he has criticised.

His former finance minister Euclid Tsakalotos on Saturday warned that cooperating with parties "who have built their political power on clientilism" would be difficult.

"I don't see how we can change public life and combat tax evasion with such alliances," Tsakalotos told the liberal Kathimerini daily.

A 'costly experiment'

Conservative leader Meimarakis hit back in an interview on Saturday, dismissing Tsipras's seven months in government as "an experiment that cost (the country) dearly."

Tsipras won office in January on an anti-austerity ticket but then upset supporters in July with a U-turn cash-for-reforms deal struck with Greece's international creditors, despite a huge "no" vote in a referendum on the issue.

The left-wing government had earlier shut banks to avert a deposit run and imposed capital controls that are still felt in the economy.

Meimarakis warned voters against re-electing a man who has publicly admitted to opposing the bailout he signed.

"Do you know of any other prime minister who brokered a deal, brought it to parliament, voted for it and signed it, whilst saying he does not believe in it?" the conservative chief told To Vima weekly.

"I fear that if Syriza is elected... the country will soon be led to elections again, and this would be disastrous," said Meimarakis, a 61-year-old former defence minister.

Polls open at 0400 GMT on Sunday and close exactly 12 hours later.

Even after the broken promises, many voters believe Tsipras acted honestly and with their interests at heart -- a break with past leaders they perceived as corrupt and linked to powerful interests.

'Shifting sands'

With regard to opinion surveys, Greek pollsters are calling for caution after polls failed to call the result of a July referendum on austerity.

"It's the first time I've felt unable at this point to make a forecast," political columnist Paschos Mandravelis told AFP.

Polling institutes are working "in shifting sands, after seven months of major political upheaval", said Thomas Gerakis at the Marc Institute.

In January, Syriza won office with 36.3 percent of the vote, but Tsipras resigned in August, leading to this weekend's election, with anti-euro hardliners quitting to form a new party, stripping Syriza of a fifth of its members of parliament.

On Friday his flamboyant one-time finance minister Yanis Varoufakis said he would vote for the rebels in Sunday's election.

The vote is the third this year for Greeks including the referendum, and the fifth in six years as Tsipras seeks a fresh mandate to push through the tough reforms he pledged under a new 86-billion-euro ($97-billion) international rescue in July.

In a flurry of interviews, he has defended his decision to put the country above his party, saying that had he refused to agree the three-year bailout, Greece would likely have been ejected from the eurozone.

On Friday, he told Antenna TV that he would "tug the rope" in order to secure relief on Greece's huge national debt from EU creditors in the coming months.