WASHINGTON -- The first pope from the Americas is ready to elevate to sainthood an 18th-century missionary who brought Catholicism to what is now the state of California.

Pope Francis planned to canonize Junipero Serra during a Mass on Wednesday outside the Basilica of the National Shrine of the Immaculate Conception in Washington, the first canonization on U.S. soil.

The pope announced in January that Serra, a Franciscan friar from Spain revered by Roman Catholics for his missionary work, would become a saint. Francis fast-tracked his canonization by eliminating the need to show proof of two miracles. Serra was beatified in 1988.

The canonization is controversial, especially in California, where Native Americans say he wiped out indigenous populations, enslaved converts and contributed to the spread of disease.

Francis was to celebrate the Mass in Spanish in front of a congregation of 25,000, including some Hispanics from California. Washington's archbishop, Cardinal Donald Wuerl, said the Mass would give Francis, a native of Argentina, an opportunity to highlight the contributions of Hispanics to the nation and the church.

Serra introduced Christianity to much of California as he marched north with Spanish conquistadors, establishing nine of the state's 21 missions. The Vatican has defended his record, saying it shows he worked in defence of Native Americans.

Many Latinos in the U.S. view the canonization of a Spanish-speaking missionary as a badly needed acknowledgment of the Hispanic history of the American church, and as an affirmation of Latinos as a core part of the U.S. Catholic future. Latinos make up about 38 per cent of U.S. Catholics, but are well above the majority in several dioceses. The Archdiocese of Los Angeles, the largest U.S. diocese, is about 70 per cent Latino.

During a visit to South America in July, Francis apologized for the sins, offences 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.

The mission, founded in 1771, also was preparing for a silent protest by members of the Ohlone Costanoan Esselen Nation, who intended to pray for ancestors buried in the mission's cemetery.

AP Religion Writer Rachel Zoll in New York and Associated Press writer Gillian Flaccus in Santa Ana, California, contributed to this report.