Hundreds of thousands of Sri Lankans have crowded the Colombo seafront as Pope Francis canonised the island's first saint in one of the biggest public gatherings the city has ever witnessed.

On Wednesday, Francis conducted a mass on Colombo's Galle Face Green before canonising Joseph Vaz, a 17th century missionary who disguised himself as a beggar, in the first papal visit to the island nation in two decades.

It is the highest-profile celebration at the landmark site since former President Mahinda Rajapaksa led a victory parade in 2009 after the end of the country's brutal decades long civil war with Tamil rebels.

The pope, who has focused on post-war reconciliation during his visit, said Vaz had shown "the importance of transcending religious divisions in the service of peace", ministering to those in need regardless of their creed.

Sri Lanka is a mainly Buddhist country but has significant Hindu, Muslim and Christian minorities, and has witnessed a rise in religious violence in recent years.

"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 convictions, free from intimidation and external compulsion," said the pope.

Vaz is credited with reviving the Catholic church on the island at a time of religious persecution by Dutch colonisers, giving him a contemporary significance for Sri Lankans.

He travelled from village to village ministering to Catholics from both the Tamil and the majority Sinhalese ethnic groups, disguised as a beggar because the Dutch had banned Catholic priests from the island.

The Our Lady of Madhu church in the mainly Tamil north provided sanctuary during the war, and is now a pilgrimage destination for Christians from across the ethnic divide.

The pope's visit comes just days after an election that exposed bitter divisions on the island and saw the surprise exit of former president Rajapaksa.

Around six percent of the mainly Buddhist Sri Lanka's 20-million-strong population is Catholic, including people from both the Tamil minority and the majority Sinhalese ethnic groups.

Source: Agencies