The former vice president said this during the National Executive Committee (NEC) meeting of the PDP in Abuja on Tuesday, February 19, 2019.
Atiku accused his opponent in the 2019 presidential election of only pretending to be a democrat so he could return to power and wield it irresponsibly.
Buhari was a military Head of State between 1983 and 1985 before he was elected democratic president in 2015 after vowing that he's a reformed democrat.
Atiku said, "I have always said that General Buhari is not a democrat, he's never been a democrat. We started the fight to drive the military, he was never there, he never participated. And you can see that because he just wore the garb of democracy so that he can come back to power and do whatever he's doing today.
"He's not a democrat, it's a lip service. He's more of a power monger than a democrat," he said.
Atiku's comment comes just a day after the president's controversial comment about directing security operatives to be "ruthless" in dealing with ballot box snatchers who attempt to disrupt the election.
"Anybody who decides to snatch boxes or lead thugs to disturb it, maybe that's the last unlawful action you'll take. I really gave the military and the police order to be ruthless," President Buhari said during his own party's caucus meeting on Monday.
However, in reaction to the president's directive, Atiku urged security operatives to not carry out any unlawful orders even if they come from the office of the Commander-in-Chief.
"In military, commanders and officers are not bound to execute orders that are manifestly unlawful. This is the law, this is the tradition in the military.
"If you're a professional military commander, officer and soldier, and also a policeman, you're not bound to execute an order that's manifestly unlawful no matter who issues that order," he cautioned.
Atiku and Buhari are favourites to win the 2019 presidential election which was recently rescheduled from its initial February 16 date to February 23.
71 other candidates are listed to contest in the election even though some of them have dropped out and declared support for Atiku or Buhari.
The leading candidates in the chasing pack are Kingsley Moghalu of the Young Progressive Party (YPP), Omoyele Sowore of the African Action Congress (AAC), and Fela Durotoye of the Alliance for New Nigeria (ANN).