Colombo: The Pope canonized 17th Century missionary Joseph Vaz at the sea-front service in Colombo.
It is the first papal visit since the end of the war in 2009 that saw the army and rebels accused of atrocities.
On Tuesday, the Pontiff called for the “pursuit of truth” to promote “justice, healing and unity” after years of war.
He arrived early for the Mass on Wednesday, informally greeting worshipers, reported bbc.com.
Hundreds of thousands showed up for the service at Galle Face Green, with many lining up from Tuesday to secure a place.
In keeping with his message of unity for Sri Lanka, Pope Francis urged its citizens to follow the example of Joseph Vaz and learn to overcome religious differences.
Saint Joseph embodied a challenge to “testify to the gospel message of reconciliation to which he dedicated his life” and showed “the importance of transcending religious divisions in the service of peace”, he said.
He added: “Religious freedom is a fundamental human right. Each individual must be free alone or in association with others to seek the truth and to openly express his or her religious conviction.”
The canonised saint, Joseph Vaz, was an Indian-born missionary who served Sri Lanka’s Catholic community, who were persecuted by Dutch colonists. He has been credited with re-establishing the Catholic Church on the island.
The BBC’s Yogita Limaye in Colombo says there has been a great deal of enthusiasm not just from the Catholic community of Sri Lanka but also from India.
But his canonisation has also provoked concern from some in Sri Lanka, where the majority of the population is Buddhist.
Susantha Goonatilake, president of the Royal Asiatic Society of Sri Lanka, told Reuters news agency that Pope Francis should apologise for the Church’s destruction of Buddhist temples in its early years.
Pope Francis’ visit, part of a six-day tour of Asia which will also see him visiting the Philippines, comes at a significant time for Sri Lanka, which elected its new president Maithripala Sirisena last week, ending 10 years under Mahinda Rajapaksa.
Later Pope Francis will speak at prayers in Madhu in the north – a region which saw some of the fiercest fighting of the 26-year war.
The conflict, arising from ethnic tensions between the majority Sinhalese and the Tamil minority, ended in 2009 when the army defeated separatist Tamil rebels. The United Nations said both sides committed atrocities against civilians.
The government consistently denied allegations that it was responsible for the deaths of many thousands of civilians in the final phase of that war. Last year the UN approved an inquiry into alleged war crimes.
Just over 7% of Sri Lanka’s population are Christian, mostly Catholic – but they include both Sinhalese and Tamils. About 70% of Sri Lankans are Buddhist, with 13% Hindus and 10% Muslims.
The last papal visit was 20 years ago, when Pope John Paul II was boycotted by Buddhist leaders. But on Tuesday, Pope Francis met a group of Buddhist, Hindu and Muslim leaders, urging reconciliation.
His visit comes amid change in Sri Lanka, where Maithripala Sirisena took office as president on Friday, replacing Mahinda Rajapaksa.
Critics said Mr Rajapaksa’s leadership had been marred by increasing corruption and authoritarianism.
There have been early signs of reform under the new leader, but it is not yet clear if his approach to addressing the legacy of the war will differ from his predecessor, who is seen as a hero by many Sri Lankans for ending the conflict.