"The opposition must form a granite alliance to defeat the sinister intentions of a few," Buhari wrote, in reference to a purported plot by the PDP to rig the elections.
In the letter dated April 4 and reprinted in Monday's newspapers, he invited all other opposition presidential candidates to a meeting in the capital Abuja on Wednesday.
The elections are seen as a test for Nigeria's democracy as they should produce the country's first transition from one elected leader to another since independence in 1960.
The election of Olusgun Obsanjo as president in 1999 ended nearly 30 years of continuous dictatorship in the oil-rich country.
The consititution prevents from Obsanjo standing for a third term but many observers have accused him of reluctantly relinquishing power voiced concerns the government is meddling in the process to ensure the PDP holds onto power.
Civil society groups are also worried by poor preparations for the vote and rising political violence.
However the unified opposition to the PDP that Buhari craves is proving elusive. He has signed an alliance with the Action Congress, the second-largest opposition party but no details of how the alliance would work practically have been resolved.
It is unclear who AC voters would choose should the party's desired candidate, the country's vice president, Atiku Abubakar, be unable to stand.
|Buhari is reported to be leading in polls [AP]|
Abubakar, has been disqualified from running because of an indictment for fraud which he is contesting in the courts.
Abubakar has kept campaigning in the hope that the Supreme Court will overturn his disqualification before the presidential vote on April 21.
An opinion poll conducted by the US state department in January showed that Buhari was favourite to win the largest number of votes, although many analysts are predicting a close run, possibly going into a second round.
There are a total of 24 candidates running for president but Buhari is by far the most credible challenger to the PDP's Umaru Yar'Adua.