Media playback is unsupported on your device Media caption The canonisation of Serra is opposed by Native Americans

Pope Francis is the fourth pontiff to visit the US, but he'll perform the first canonisation ever on US soil. So was 18th Century missionary Junipero Serra a champion of Catholicism or a brutal instrument of colonisation?

Born in 1713 on the Spanish island of Mallorca, Serra was a Franciscan friar who spent much of his life spreading the Catholic faith in the western part of what is today the United States. It was his work there that has generated controversy.

While Serra enjoys popularity among many and was beatified by John Paul II in 1988, the missionary remains highly controversial with some groups, especially Native Americans in California.

They claim that Serra was instrumental in the obliteration of indigenous populations, as well as several human rights abuses including enslavement.

Descendents of these original Californians have asked the Pope not to canonise the friar, claiming he acted more like a religious conqueror than a saint.

Still others say that Serra, like Pope Francis, was a champion of the poor and downtrodden, and that Christianity in much of California could be largely attributed to his work.

Marching aside the Spanish conquistadors, Serra established eight of the state's 21 missions. His proponents - including the Vatican - say that it was during this time he worked as a moderating force on the colonial conquerors and defended the rights of the Native Americans.

Image copyright Reuters Image caption An image of Junipero Serra is on display at Mission San Diego de Alcala in San Diego, California

Despite the controversy, in January, the Pope announced that Serra would become a saint, after bypassing the standard requirement that two miracles could be attributed to the missionary.

Since then the Pope has worked to ameliorate some of the concerns. During a South American tour in July, Pope Francis apologised for the crimes committed against indigenous peoples at the hands of the church, but he did not mention Serra by name.

The canonisation is slated to take place in front of an audience of 25,000 people in Washington.

Notably, in a sign of the shifting demographics of the Catholic church in the Western Hemisphere, the Mass will be celebrated in Spanish.

Read a profile of Junipero Serra in Spanish (BBC Mundo)

Many of those in attendance will be Hispanic, which Washington's archbishop, Cardinal Donald Wuerl, said would give the Argentine-born pope a chance to pay homage to the contributions of Hispanics to the US and to the Catholic church.

At the Carmel mission in California, the site of Serra's death, as many as 400 worshippers are planning to watch the mass via video stream.

But at the same site, members of the Ohlone Costanoan Esselen Nation were planning a silent protest and prayer service for ancestors buried on the mission's grounds.