Pope John Paul II, on the third day of his visit to Canada, today beatified a Canadian-born nun, called for more lay involvement to shore up the declining ranks of the clergy, and challenged young Catholics to remain constant in their faith.

Thousands from all over Quebec stood for hours in a chilly drizzle to cheer the tightly guarded pontiff as he passed in his specially built vehicle, and thousands of schoolchildren chanted "Vive le Pope," at him in Notre Dame Cathedral.

About 350,000 persons gathered for an open-air mass this morning in Jarry Park, where the Montreal Expos played before the Olympic Stadium was built.

The service began with the beatification, the first step toward canonization, of Canadian-born Sister Marie-Leonie Paradis, who founded the Congregation of the Little Sisters of the Holy Family. Members of the order keep house for priests.

Pope John Paul called it a "special kind of commitment" and said the nun's honor "reminds us how much religious communities in every sector of ecclesiastical and social life have contributed to Canada. They have done this through contemplative prayer, education, assistance to the poor, work in hospitals and apostolic involvement of all kinds."

Like their counterparts in the United States, many Canadian Catholic women have been pressing, with some success, for a greater role in the church.

Quebec's bishops, in their once-in-five-years report to the Vatican last fall, set as one of their priorities the granting of a more equitable role in the church for women.

The commitment of the Canadian bishops to women's rights is credited with shattering one sex barrier during the pope's visit. For the first time during a papal tour, women are helping distribute holy communion at papal masses, just as they do in thousands of Canadian parishes every Sunday. American women were denied that role during the 1979 papal tour of the United States.

In his early-morning address to several thousand cheering, applauding clergy of Quebec Province, the pontiff strongly endorsed "coresponsibility of laity and priests, not only to assist the clergy . . . but, because it is the role of baptized and confirmed lay persons to participate . . . , in the progress of the church."

He touched on his now familiar ban on priests in politics by saying, "If the church is to have a social role, that role must of necessity be played by the laity," inspired by the priests and bishops.

The pontiff has steered clear of Canadian politics thus far except for his endorsement yesterday during a mass at Ste. Anne de Beaupre, of Indians' right of self-determination.

He commended the Indians' desire to "control your future, to preserve your cultural traits, to establish an educational system where your languages are respected" -- issues the Indians are seeking to negotiate with the Canadian government.

Security has been massive in this city, where just a week ago three people were killed in a terrorist bombing that appeared to be related to the papal visit. Jittery police take seriously the danger of "copycat" bomb threats that often follow such incidents.

Yesterday, a branch of the subway was closed briefly when a "suspicious" parcel was reported in a station. The parcel was found to contain stale popcorn. Here in Quebec, the pope has been speaking French. But he has also acknowledged the 15 percent of Montrealers who speak English by occasionally addressing them in their language.

In his address to the clergy, whose rapidly thinning ranks have left a ratio of one priest for every 2,150 faithful in Quebec, the pope acknowledged the "deep-seated process of change" and "crisis of values" confronting the church here as well as in other western countries.

"You must rise to the challenge," he admonished his priests, telling them they must not allow themselves to be paralyzed. "We must, more than ever, see to it that the voice of Christianity has a right to be heard in this country," he said.

Following a late-afternoon meeting with 2,500 schoolchildren in Montreal's Notre Dame basilica, the pontiff completed his visit here with a rally for more than 60,000 young people at the Olympic Stadium. The rally featured gymnasts, mimes, ballet dancers and a symbolic release of white doves.

"I have come to invite you to open your eyes to the light of life, to Christ Jesus," he told the youth.

There are, to be sure, problems, he said, but added, "You must remember this in moments of doubt: If you follow Christ you will develop your full potential."

"In times of darkness, do not seek an escape. Have the courage to resist the dealers in deception" -- drugs and alcohol -- "who make you pay dearly for a moment of artificial paradise," he told them.

For those who have problems with the church, he acknowledged to the cheering young people that "it is still far from being the perfect mirror that would reflect the whole face of Christ." But "take your place in it," he pleaded, "however imperfect it remains."

"Remain seekers after truth," he concluded. "Use with courage the rich gifts that are within you. Give yourselves without reserve to the service of justice, of peace, of freedom, and of love in the light of Christ."