The six, who are part of a larger grouping of opposition candidates holding almost daily protests for weeks, said they will not take part in Thursday's vote and will urge their supporters not to cast their ballot.
"We will call (on people) not to go and vote," said presidential hopeful Marc Ravalomanana, 73, one of two former presidents to join calls for a boycott.
The Indian Ocean island nation has been shaken by a fierce battle between President Andry Rajoelina, who is running for re-election, and most opposition leaders.
Almost all the 12 opposition candidates in the running have led near daily, unauthorised marches in Antananarivo, for more than a month, protesting at what they have called an "institutional coup" that favors the incumbent.
The demonstrations that have in average drawn a few hundred supporters have been regularly dispersed by police.
On Saturday security forces fired teargas and detained 11 protesters at yet another rally.
"I will not stand for election, that is very clear," said Hery Rajaonarimampianina, 65, another former president who was in the running to replace Rajoelina.
Last week, the head of the lower house of parliament, who leads a mediation group to find a way out of the crisis, called for the suspension of the presidential elections, to ensure "peace" and "harmony."
"The situation is deleterious. There is no calm, it is not the time to hold elections," said candidate Auguste Paraina, 69. The opposition grouping supports a suspension of the electoral process.
But a spokeswoman for Rajoelina called the request a "far-fetched idea."
The outgoing president held his last campaign rally in Antananarivo on Sunday in front of a fervent crowd of several thousand people donning t-shirts bearing his image.
"I'm going to win, that's for sure, and in the first round," he told AFP in an interview at the weekend.
'Rigged vote'
Rajoelina, 49, became Africa's youngest head of state in 2009 when he took power on the back of a coup.
After not running in the 2013 election due to international pressure, he was voted back into power in 2018 and has since held the reins in a country that remains among the poorest in the world despite vast natural resources.
The show of support for the president at the weekend "means nothing" said Roland Ratsiraka, 57, another opposition candidate, hinting at the common practice of paying people to attend political rallies in the country.
The European Union, the United States and other members of the international community have expressed "deep concern" at the political tensions and denounced the excessive use of force against the opposition.
A source at Madagascar's electoral commission said some irregularities have already been reported, including some would-be voters being pressured not to register.
The crisis erupted in September after Rajoelina resigned in line with the constitution in order to run for re-election.
The president of the Senate was supposed to take over but declined for "personal reasons," leaving the task to a "collegial government" headed by the prime minister, an ally of Rajoelina.
The move was accepted by the Constitutional Court, which also dismissed appeals to have Rajoelina's candidacy declared void over his dual French nationality, sparking opposition anger.
Detractors believe that under local laws the President was to lose Madagascan nationality and with it the ability to lead the country because of the naturalisation, which was revealed by the media in June.
In Itaosy, an eastern suburb of Antananarivo, a few hundred people joined a peaceful opposition march on Monday.
"It's a rigged vote," said Sandra Rafarasoa, 33, standing outside her modest repair shop for two-wheelers.
Voters in Madagascar were initially due to head to the polls on November 9, but the top court in October ordered that the elections be postponed after a presidential candidate was injured during a demonstration.
The date for a potential second round vote on December 20 was kept unchanged.
Thursday's election "will not take place", said Hajo Andrianainarivelo, 56, another opposition candidate.
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