With church bells ringing joyously and thousands cheering, Pope Francis arrived late Wednesday afternoon on the grounds of The Catholic University of America to lead his first Mass in the United States.
Shouts of "Papa Francisco!" rose as the pontiff waved to the throngs from the popemobile as it arrived in a motorcade at the Washington campus.
The Mass, delivered in Spanish, is being held outside the Basilica of the National Shrine of the Immaculate Conception. In the ceremony, Francis is conferring sainthood on Junipero Serra, an 18th-century Spanish missionary who spread the word of the church in what is now California.
About 25,000 people were expected to attend, with several thousand tickets reserved for Hispanics from California.
WATCH LIVE: Papal Mass of Canonize For Junipero Serra
The basilica is the nation's largest Roman Catholic church. Organizers anticipated as many as 3,600 seminarians and men and women novices at the service. Scores of religious in white robes filled the front rows of the upper church.
In the unticketed overflow area, hundreds sat on the grass or sought shade in the warm sun. Groups of Spanish speakers and nuns in habits watched the ceremonial proceedings on the giant screen, keeping a respectful quiet.
In January, the pope announced that Serra, a Franciscan friar from Spain revered by Roman Catholics for his missionary work, would become a saint. Serra introduced Christianity to much of California as he marched north with Spanish conquistadors, establishing nine of the state's 21 missions.
Francis fast-tracked Serra's canonization by eliminating the need to show proof of two miracles. Serra was beatified in 1988.
A holy card for Junipero Serra, the Franciscan friar who is to be canonized by Pope Francis at the Basilica of the National Shrine of the Immaculate Conception in Washington, Sept. 23, 2015.
However, Serra's elevation to sainthood is a sore point for many Native Americans, who say the friar contributed to the abuse and elimination of many indigenous tribes from enslavement and disease.
The pope has described Serra as one of the founding fathers of the United States and called him "a saintly example of the church's universality and special patron of the Hispanic people of the country."
Archbishop of Washington Cardinal Donald Wuerl said the Mass would give Francis, a native of Argentina and the first pope from the Americas since 741, an opportunity to highlight the contributions of Hispanics to the nation and the church.
During a visit to South America in July, Francis apologized for the sins, offenses and crimes committed by the church against indigenous peoples. He did not mention Serra.
"Serra's canonization provides an opportunity to reflect on those sins, and resolve not to repeat them,” said Kathleen Sprows Cummings, director of the University of Notre Dame's Cushwa Center for the Study of American Catholicism.
But the pope's apology did little to quiet those who oppose the canonization.
"We believe that this canonization is going to backfire," Valentin Lopez, chairman of the Amah Mutsun Tribal Band, said Tuesday. "This has woken up the outrage of indigenous people around the world."
In California, at the historic Carmel mission where Serra died, up to 400 faithful planned to watch the Mass on a giant screen in the courtyard.
Earlier Wednesday, President Barack Obama and Pope Francis saluted each other in speeches that were frequently interrupted by applause from the 15,000 guests watching from the White House's South Lawn.
During a speech delivered in English, Francis appealed to the United States to take urgent action and help slow the pace of climate change, because the world is at "a critical moment of history."
Obama warmly welcomed the leader of the Roman Catholic Church, agreeing that Americans and people throughout the world have "a sacred obligation to protect our planet."
U.S. President Barack Obama (R) sits with Pope Francis during an arrival ceremony for the pope at the White House in Washington, Sept. 23, 2015.
After the brief statements, the two leaders held a private meeting in the Oval Office, speaking for about 40 minutes.
Obama also gave the pope two gifts: a sculpture of an ascending dove, an international symbol of peace as well as the Christian symbol for the Holy Spirit; and a key from the Maryland home of Elizabeth Ann Seton, the first native-born American to be declared a saint.
After meeting with Obama, Francis led a worship service with America's 450-strong bishops' conference at the Cathedral of St. Matthew the Apostle. He told the bishops that clerics' crimes of sexual abuse of minors should never be repeated.
While the pope did not utter the words "sexual abuse," he referred to the scandal by talking about "difficult moments" and providing help for victims.
On Thursday, Francis becomes the first pope to deliver an address before a joint meeting of Congress.
He will travel later in the day to New York City, where he will celebrate Mass at St. Patrick's Cathedral.
On Friday, he will address the United Nations General Assembly.
On Saturday morning, he'll travel to Philadelphia for this weekend’s Vatican-sponsored World Meeting of Families.
On Sunday, he will celebrate an outdoor Mass, which is expected to draw nearly 2 million people.
Some material for this report came from AP.