Pope Francis spoke up for the victims of abortion and for those caught up in the immigration crisis in his strongest speech so far since arriving in the US.

In an address to US Catholic bishops, he said: "The innocent victims of abortion, children who die of hunger or from bombings, immigrants who drown in the search for a better tomorrow, the elderly or the sick who are considered a burden, the victims of terrorism, wars, violence and drug trafficking, the environment devastated by man's predatory relationship with nature – at stake in all of this is the gift of God, of which we are noble stewards but not masters. It is wrong, then, to look the other way or to remain silent."

Of the many thousands of words he will speak over the next few days in Spanish and English, this one paragraph effectively summed up the heart of his mission to the United States where he has met with a rapturous welcome. 

The Pope, who is 78, looked exhausted as he arrived at he arrived the Basilica of the National Shrine of the Immaculate Conception in Washington DC for the first canonisation on American soil, of Spanish Friar Junipero Serra, who converted indiginous peoples in California to Christianity in the 18th century. The canonisation is controversial because the ministry of Serra is regarded by some contemporary commentators as far from saintly. It generated a Twitter hashtag of its own, #SerraisnoSaint.

There were also protests on Twitter at how the all-male processions of bishops and priests during services highlighted the male-only hierarchy of the Catholic Church, and protests from the Women's Ordination Worldwide group outside St Matthew's Cathedral in Washington. The women, with placards stating "All are equal in God's eyes" and "Ordain Women", have made plans to accompany him throughout the visit, including to New York tomorrow and then Philadelphia at the end of the week. 

One highlight of the day was when the Pope stopped unexpectedly in his popemobile to greet a five-year-old girl, Sophie Cruz, the daughter of illegal immigrants, who handed him a letter pleading her case. 

"I want to tell you that my heart is sad," she said in the letter which she had memorised in English and Spanish. "I believe I have the right to live with my parents, I have the right to be happy.

"All immigrants just like my dad help feed this country. They deserve to live with dignity. They deserve to live with respect. They deserve an immigration reform."

Immigration is expected to be among the issues raised by Pope Francis in his address to Congress tomorrow.

In an exclusive comment for Christian Today, Caroline Wyatt, who is in the US covering the visit for the BBC, said: "Washington is not a town normally impressed by VIP visits. But it seems that Pope Francis is different. Federal workers have been given a day off, due to the tight security surrounding the Pope, and his every route has been lined by enthusiastic, cheering crowds. Many have been impressed with the way this Pope lives his message about poverty and taking care of the environment: his black Fiat 500 is not the usual sort of limousine used by VVIPs."

She added: "However, there is a certain nervousness here in Washington about what he might say at Congress, when he becomes the first Pope to speak to US lawmakers. Some have protested against that invitation, saying that a religious leader had no place speaking to democratically-elected body of representatives. Others wonder what he will focus on in his speech, with some hoping for more on migration, the environment and social issues – and others wanting less."

She said he was in the US to send a message to the Catholic faithful - "that the face of Catholicism in the US is changing, and that they need to embrace that, and the migrants who are bolstering congregations in the west and the south are to be welcomed, while the Church here needs to come together across the divides over sexuality, abortion and many other issues that split both America and its Catholics. He also wants to send a clear message to the world's richest nation: that it needs to focus more on helping to save the environment, and stop damaging the world we all share, and that that is a task that cannot wait."

The Pope is well aware of the political pitfalls of all the topics he is addressing, from migration to the environment to the child abuse scandals that have plagued the Roman Catholic church, and he has chosen his words carefully throughout this trip, she said. "Though he did joke on the plane to the US about reports in parts of the US media questioning whether he really was Catholic because of some of his comments on the economy. He jokingly offered to recite the Creed just to prove it. None of the journalists on the Vatican plane took him up on the offer.... "