The February 14 election, in which Buhari is challenging President Goodluck Jonathan, is too close to call, leading to a possible run-off if neither candidate secures a first-round majority.
Buhari, dismissed the suggestion of a tight race, saying in an interview: "What's their reason for 'too close to call'? What is their speculation on? I'm expecting a landslide victory."
The former military head of state, who ruled for 20 months after seizing power in December 1983, has stood unsuccessfully for the presidency three times in the last 16 years.
On each occasion, in 2003, 2007 and 2011, he alleged electoral foul-play.
When asked if he would accept defeat this time around, the former military head of state replied: "I'm not going to lose. So, I won't answer that question."
Nigeria's main opposition party is considered to have the best chance yet of ending the Peoples Democratic Party's (PDP) unbroken run in power since the country returned to civilian rule in 1999.
Buhari, who was noted for his hard line approach to indiscipline and corruption during his time in power, has been touted as the answer to Nigeria's lengthy list of problems.
The PDP has sought to portray Buhari as yesterday's man with dangerous, outdated ideas, but he asserted that Africa's most populous nation and leading economy was ready for change.
"I have visited 34 states so far. In each of the states there are three things that are consistent, that are fundamental issues to this country — insecurity, the destruction of the economy and corruption," he said.
"Every Nigerian is feeling worried."
Buhari further rejected claims from the PDP that he would rule again with an iron fist, saying the circumstances under a military regime were different from civilian rule.
"Multi-party democracy and the three tiers of government, enshrined in the constitution, had to be respected, with consequences for anyone acting outside the system," he said.
The APC has portrayed President Jonathan as weak on security and of being unable -- some say unwilling -- to end the Boko Haram insurgency in the northeast.
Some soldiers have complained that they lack the weapons and even bullets to fight the better-armed militants, despite defence spending accounting for some 20 percent of the federal budget.
Buhari said that if elected, his administration would "empower the law enforcement agencies to be much more efficient".
"The first thing we will do is to make sure there is efficient utilisation of resources. If money is voted for equipment and training, that money will go to equipment and training," he added.
The APC has claimed that Jonathan has failed to tackle endemic graft and Buhari said corruption was practically "a culture" in the country.
"I believe the most serious thing to do is to draw a line, that you intend to move forward," he said but added that investigating every allegation of corruption would be a hindrance to progress.
"From the day we are sworn in as a government, anybody who abuses trust will be called to account," he added.
- AFP