Pope Francis on Sunday reaffirmed Catholic opposition to gay marriage as he opened a three-week gathering of bishops from around the world but said the Church had to show love and understanding towards all.

Francis presided at a solemn Mass in St. Peter's Basilica to open the meeting, known as a synod, on the theme of the family in the modern world.

But the run-up to the synod, attended by some 300 bishops and other delegates, has been dominated by gay issues.

On the eve of the gathering, the Vatican dismissed a Polish priest from his Holy See job after he came out as gay and called for changes in Catholic teachings against homosexual activity.

Conservative Catholics held a conference in Rome just before the synod started on how homosexuals can live by Church's rules that they should be chaste while Catholic gay activists held another demanding full acceptance of active gays in the Church.

Francis dedicated one third of his homily to the topic of love between man and woman and its role in procreation.

"This is God's dream for his beloved creation: to see it fulfilled in the loving union between a man and a woman, rejoicing in their shared journey, fruitful in their mutual gift of self," he said.

He also spoke of the "true meaning of the couple and of human sexuality in God's plan," a clear reference to heterosexual marriage.

MERCIFUL CHURCH

But Francis also stressed that the Church must be more welcoming, charitable, compassionate and merciful to all people, particularly those whose lives have been wounded and who those find it difficult to adhere to all of the Church's regulations.

The leader of the 1.2 billion member Church said the person "who falls or errs must be understood and loved."

"The Church must search out these persons, welcome and accompany them, for a Church with closed doors betrays herself and her mission, and, instead of being a bridge, becomes a roadblock," he said.

In its explanation of the firing of the Polish monsignor on Saturday, the Vatican said his very public coming out was intended to put undue media pressure on the synod on gay issues, which are expected to be only a small part of the bishops' discussions.

The story made the front page of nearly all Italian newspapers, with one headline calling it "An Earthquake in the Vatican".

At a preliminary synod last year, bishops watered down a initial statement that was seen as a major change of tone toward homosexuals. That statement spoke of "gifts and qualities" of homosexuals but was changed after a backlash by conservatives.

One key topic at the synod will be how to reach out to Catholics who have divorced and remarried in civil ceremonies.

They are considered by the Church to be still married to their first spouse and living in a state of sin. Some bishops want a change to the rules that bars them from receiving sacraments such as communion.

Last month, Francis made it simpler and swifter for Catholics to secure a marriage annulment, the most radical such reform for 250 years, and told bishops to be more welcoming to divorced couples.