The fourth annual demonstration began at historic Olvera Street, where Archbishop Jose Gomez of the Catholic Archdiocese of Los Angeles and other religious leaders delivered remarks.
Gomez then led a massive crowd of marchers to Los Angeles State Historic Park in Chinatown, where a program featuring speakers and musical performances was held.
Among those scheduled to address the crowd were actress Patricia Heaton; Karen Gaffney, the first person with Down syndrome to swim the English Channel; and former gang member Jose Arellano of Homeboy Industries.
"OneLife L.A. means sharing the love of God with others -- especially those who are poor and forgotten, and those who are alone and excluded," the archbishop said before the event.
The theme for the march was "Made for Greater" and drew people from all over Southern California.
City News Service contributed to this report.