"Brethren, we do not honor the priesthood of God if we are not kind to others," he told the crowd of men and boys in the Conference Center in downtown Salt Lake City.
He asked his listeners to self-reflect, and put themselves in a better position to "call down the powers of heaven for ourselves, for our families and for our fellow travelers in this sometimes difficult journey back to our heavenly home."
The increasingly frail Monson delivered the first sermon of the priesthood session. Traditionally, the church president gives the final talk of that meeting.
Viewed by Mormons as a "prophet, seer and revelator," Monson attended the Saturday morning session, but did not go to the afternoon meeting, choosing to conserve his strength for the rest of the conference, which concludes Sunday.
He quoted verses from the faith's foundational scripture, the Book of Mormon, regarding the principle of Christlike love: "If ye have not charity, ye are nothing, for charity never faileth."
Dieter F. Uchtdorf, Monson's second counselor in the governing First Presidency, spoke of humility and gratitude in service while cautioning LDS leaders of every rank and stripe about how they wield their authority.
"There is nothing wrong with wanting to serve the Lord," he said, "but when we seek to gain influence in the church for our own sake in order to receive the praise and admiration of men we have our reward."
Uchtdorf recalled words he'd been told as a new general authority by the late LDS apostle James E. Faust about the relationship between Mormons and their leaders with lofty positions.
Members treat LDS authorities kindly and say nice things about them, Faust told his younger charge, but added, after a pause, "Dieter, always be thankful for this, but don't you ever inhale it."
Church leadership, Uchtdorf said, is "not so much about directing others as it is about our willingness to be directed by God."
He paraphrased Jesus' words to James and John, two "rough-around-the-edges" apostles in the Bible. Uchtdorf said Christ essentially reminded those "Sons of Thunder" that "you can't get honor in the kingdom of heaven by campaigning for it. Nor can you power-lunch your way to eternal glory."
The LDS leader urged his listeners to love all God's children "regardless of their socioeconomic circumstance, race, language, political orientation or nationality."
Monson's first counselor, Henry B. Eyring, encouraged Mormon priesthood holders to see others as "clean, bright, noble" children of God.
Eyring called the priesthood a spiritual gift through which Mormon boys and men can serve others.
He said God "notices and will support" those who try to do his will. Heaven's work is "not just to solve problems; it is to build people," Eyring said, adding that priesthood service "will change the way you look at others."
"If you feel a little overwhelmed, take that as a good sign," he said. "It indicates that you can sense the magnitude of the trust God has placed in you. It means you have some small understanding of what the priesthood really is."
Editor David Noyce contributed to this story.