At a joint news conference, the former head of state, Henri Konan Bédié, and the former prime minister, Pascal Affi N’Guessan, called on their supporters to block what they described as an electoral coup.,
Mr Ouattara is standing for a third term on 31 October, which his opponents say is illegal.
Their declaration – on the first official day of campaigning – has raised fears of major political unrest.
Electoral violence when President Ouattara first took office in 2010 led to 3,000 deaths and half a million displaced.
The only other opposition candidate – after several others were disqualified – is the former MP Kouadio Konan Bertin.