By Isla Binnie

VATICAN CITY (Reuters) - Mother Teresa of Calcutta, a Nobel peace laureate known as the "saint of the gutters" during her lifetime, will be made a saint of the Roman Catholic Church on Sunday.

More than 100,000 pilgrims are expected to attend a service led by Pope Francis in St. Peter's Square at the Vatican to honor the tiny nun who worked among the world's neediest in the slums of the Indian city now known as Kolkata.

Her legacy fits neatly with Francis's vision of a poor church that strives to serve the poor, and the ceremony will be a highlight of his Holy Year of Mercy which runs until Nov. 8.

Mother Teresa and her Missionaries of Charity (MoC) order have been criticized both during her life and since her death in 1997, but many Catholics revere her as a model of compassion.

Thousands attended a papal audience on Saturday in the Vatican, where a large canvas of the late nun in her blue-hemmed white robes hung from St. Peter's basilica.

"Her testimony makes us reflect and transform...and make a better world," Brazilian priest Carlos Jose Nacimento said.

Critics say she did little to alleviate the pain of the terminally ill and nothing to tackle the root causes of poverty. Atheist writer Christopher Hitchens made a documentary about her called "Hell's Angel".

She was also accused of trying to convert the destitute in predominantly-Hindu India to Christianity, a charge her mission has repeatedly denied.

But Pope John Paul II, who met her often, had no doubt about her eligibility for sainthood, and put her on a fast track to elevation two years after her death instead of the usual five.

The Church defines as saints those believed to have led such holy lives they are now in Heaven and can intercede with God to perform miracles - two of which are needed to confer sainthood.

She is credited with healing an Indian woman from stomach cancer in 1998 and a Brazilian man from a brain infection in 2008.

The canonization will also be celebrated in Skopje, the capital of modern Macedonia where Mother Teresa was born of Albanian parents in 1910 and became a nun aged 16.

In Kolkata, where the first MoC mission was set up in 1952, there will be prayers, talks and cultural events, but no major ceremony.

Delegations from at least 15 national governments are expected at the Vatican.

(Reporting by Isla Binnie; Additional reporting by Iona Serrapica and Miran Jelenek, Editing by Angus MacSwan)