The move leaves only one candidate in the running, Abdelaziz Bouteflika, who is widely believed to have the support of the powerful military, the real source of power in Algeria since its independence from France in 1962.
The election on Thursday is meant to give Algeria its first civilian president since 1965 and reconcile a nation reeling from an Islamic insurgency that has killed some 75,000 people in seven years. The unrest was sparked when the military aborted the 1992 legislative elections.
Until now, candidates, authorities and even a banned Muslim fundamentalist party had seen the election as the turning point for a brutalized nation, encouraged by promises by outgoing President Liamine Zeroual that the vote would be clean.
Constitutional experts said according to the law, the vote would go on as planned, with only one candidate. There was no immediate reaction from the government or from Bouteflika.
The six other candidates said in a joint statement that they rejected in advance the eventual winner and called for peaceful protest marches on Friday.
Promises for a fair election "have not been respected on the ground, and so we decide on a collective withdrawal from the presidential election," they said after meeting Wednesday. "We will not recognize the legitimacy of this election."
Their fraud allegations centered on early voting in mobile stations set up in the vast Sahara Desert, and in special precincts set up for soldiers. Early voting was allowed for soldiers who were scheduled to work to protect polling stations.
Among their charges, the candidates said ballots were improperly handled and candidates' monitors were expelled from some polling places. They said Bouteflika supporters were allowed a rally in one town after campaigning was officially closed, and that supporters of Ahmed Taleb Ibrahimi, a candidate with Islamic sympathies, were arrested in some areas.
Earlier Wednesday, Zeroual, a retired general who is stepping down as president 18 months before the end of his five-year term, refused to meet with the candidates to discuss their grievances, saying they should file an official complaint.
Although killings have continued in the insurgency, violence has waned in many areas and until now the campaign has raised hopes for reconciliation.
It had been the first election supported by the banned Islamic Salvation Front, known as FIS, which has boycotted three elections since the 1992 vote. In that election, it had been poised to take power until the military stepped in and halted voting, triggering years of bloodshed.
For Thursday's vote, the Front had called on supporters to turn out en masse.
National reconciliation was the dominant theme of a vibrancampaign that featured American-style rallies with music, banners and funny hats.