The leader of the Greek Socialist Pasok party, Evangelos Venizelos, has warned of a rise of the far right in elections in the country on 6 May.
Mr Venizelos told a rally in Patras that voters should not allow neo-Nazis to "goose-step into Parliament with Hitler salutes".
Polls suggest the far-right Golden Dawn could win seats for the first time.
The election will be Greece's first since it sought two EU/IMF bailouts and announced deep spending cuts.
Opinion polls suggest parties opposed to austerity could make big gains. Golden Dawn (Chrysi Avgi in Greek) opposes the EU bailouts that have led to the Greek government's tough economic measures.
Last month Athens secured a second international bailout of 130bn euro ($171bn; £105bn) after reaching a deal with private creditors for a credit swap aimed at reducing its debt burden.
Mr Venizelos described the far-right party as an "extreme phenomenon", and "an offence to our history and to our parliament", Reuters newsagency said.
Opinion polls show that Mr Venizelos's Pasok party is trailing in second place to the centre-right New Democracy party. The two parties combined are set to win less than 50% of the vote.
This leaves the rest of the votes going to several groups that oppose the austerity measures.
One of Golden Dawn's proposals is to tighten Greece's borders and expel immigrants as illegal immigration has become a key issue.
On Sunday, Greece opened its first detention centre for illegal migrants at Amygdaleza, northwest of Athens.
The Deputy Mayor of Amygdaleza, Chrysanthos Kontaris, said that a lawsuit had been filed against the police and the citizens' protection minister for setting up of the camp and described the move as a "pre-election firework".