New York City Mayor Bill de Blasio went to an NYPD graduation ceremony on Monday calling for peace and unity, quoting scripture in the process.

His appearance, which garnered boos and applause simultaneously, hoped to relieve some tension between de Blasio and some New York Police Department officers, the most recent of which happened when hundreds of officers turned their backs on him while he eulogized Officer Rafael Ramos at his funeral in Queens. He referred to Matthew 5:9 in his comments to the new recruits.

"You can be part of the solution and that is a blessing," the mayor said before turning to the Bible verse. "'Blessed are the peacemakers for they shall be children of God.' We lost two peacemakers nine days ago. We lost two heroes. They were children of God for sure."

According to Aaron Katersky of ABC News, de Blasio presided over the graduation of nearly 900 cadets at a ceremony in Madison Square Garden. While there was some polite applause, there were also boos and catcalls while the mayor was being introduced.

"Let's be honest about the realities of our society," de Blasio told the cadets. "You'll confront all the problems that plague our society, problems that you didn't create."

A CBS News report indicated that someone from the crowd yelled "You did!" after the mayor made that remark, which received a round of applause.

The mayor then turned his focus on Ramos and his NYPD partner, Wenjian Liu.

"We lost two peacemakers nine days ago," he said. "They will not be forgotten -- they must not be forgotten."

According to CBS News, the high tensions between de Blasio and some NYPD officers came ever since Ramos and Liu were shot and killed in cold blood while sitting in their patrol car earlier this month. Officials from the police union blamed the mayor for their deaths, partly because de Blasio has openly shown support for people protesting grand jury decisions that failed to indict police officers who had killed Michael Brown and Eric Garner, who are both unarmed black men.

Al O'Leary, a spokesman of the 25,000-member Patrolman's Benevolent Association, elaborated to Henry Goldman of Bloomberg on why the police union was at odds with de Blasio.

"Instead of saying that, while you may be unhappy with the grand jury, you have to respect the decision, he went with 200 years of racism and advising his son, Dante, not to trust us," Leary said. "If you're not out there wearing a bulletproof vest, you cannot appreciate the betrayal they feel by those words."

Bloomberg reported that de Blasio, a self-described progressive Democrat, campaigned on a platform of restricting stop-and-frisk street encounters and promising to improve what he described as a poor relationship between the NYPD and minority communities. He was elected as New York mayor a year ago.

"Although rank-and-file officers have displayed anger toward New York leaders in the past, few mayors have won election on such a sweeping agenda of changing the department's policies and practices as de Blasio," Goldman wrote.