"The city was on a very good course, and that needs to be maintained and improved on, and he's kind of turned it around," Giuliani complained.
As recently as last week, Giuliani declined to comment on de Blasio's performance in office, but the two-term former mayor, a Republican, held little back during an interview on WNYM-AM.
De Blasio, a liberal Democrat who took office Jan. 1, is "going in the direction of emphasis on dependency, emphasis on big city government, less emphasis on a very well and very efficiently managed city," Giuliani said.
"All of that seems to be moving the city in the wrong direction," Giuliani told host John Catsimatidis, the grocery store magnate who waged an unsuccessful campaign for mayor last year.
Giuliani dismissed as "foolishness" the proposed ban on carriage horses championed by de Blasio, saying the horse-drawn rides are "one of the reasons" why tourists visit the city.
"I passed the horses the other day. I stopped and waved to them and wished them luck," Giuliani said.
The former mayor said that when he was in office he looked into the issue and found that the horses were well-treated. And, he added, "I hate to get into the psychology of horses, but the horses like to work."
Giuliani also took aim at de Blasio's emphasis on embracing public schools over charter schools.
"I helped to start the (charter) movement and then Mike Bloomberg really expanded it," Giuliani said.
The city was on a very good course ... and he's kind of turned it around.
"The idea that the mayor is at war with the charter schools ... he should be expanding charter schools, not trying to close them down."
Asked about Giuliani's blast, de Blasio spokesman Wiley Norvell said "we're proud of the agenda we’ve set on public safety, education and income inequality. Those are priorities that will benefit everyone, Rudy Giuliani included."
Giuliani and de Blasio have been on opposite sides of the fence for more than two decades, beginning when de Blasio was a staffer in Mayor David Dinkins' administration and Giuliani won office with a fierce attack on Dinkins' policies.
In Sunday's interview, Giuliani declared common ground with de Blasio on one point — de Blasio's selection of William Bratton as police commissioner.
Giuliani named Bratton his first police commissioner, but eventually forced him out in a dispute over who deserved credit for the dramatic decline in crime.
"I think he made a very good choice," Giuliani said of de Blasio. "He's an excellent commissioner."