U.S. President Barack Obama dove straight into the thorny abortion debate during a speech to the University of Notre Dame Sunday, urging both sides of the issue to open up their hearts and minds to improve dialogue.

Still, Obama said that "no matter how much we want to fudge it ... the fact is that at some level, the views of the two camps are irreconcilable."

But he said society should stop "reducing those with differing views to caricature. Open hearts, open minds, fair-minded words -- it's a way of life that always has been the Notre Dame tradition."

Those words came after anti-abortion activists threatened to disrupt Obama's appearance at the university, which in the biggest Catholic school in the U.S. and advocates a strict anti-abortion stance.

More than 200 protestors gathered outside the school to protest the decision to invite Obama to speak at the school's commencement, during which he also received an honourary law degree.

The protesters held up placards that read "Shame on Notre Dame" and "Stop Abortion Now."

One of the arrested demonstrators was Norma McCorvey, the plaintiff in the Roe vs. Wade Supreme Court decision that legalized abortion in the United States.

McCorvey is now a vocal opponent of abortion.

On Saturday, police arrested 19 people for trespassing and charged four others with resisting police after protestors marched on campus chanting pro-life slogans and carrying placards of aborted fetuses.

While protesters had originally indicated they would disrupt Obama's commencement speech by standing up and turning their backs on him, or by loudly saying the rosary as he talked, they abandoned such plans, said CNN reporter Susan Candiotti.

Instead, the protesters passed out stickers with images of crosses and babies' feet, to symbolize unborn children who are aborted, Candiotti told ۴ýnet on Sunday during a telephone interview from South Bend, Ind., where Notre Dame is located.

Obama supports abortion rights and earlier this year overturned a decision by former president George W. Bush to limit federal funding for embryonic stem cell research.

Obama's support for both abortion rights and embryonic stem cell research have angered many Christians in the United States, who believe both procedures destroy human life and should be outlawed.

Notre Dame's president, Rev. John Jenkins, has not responded to the protesters' complaints. But colleagues said Jenkins stands by his decision and has the support of the board of trustees.

"He respects people who differ, but he's resolute in his decision because he did it based on conscience and what he really believes in," said Richard Notebaert, chairman of Notre Dame's board of trustees.

Meanwhile, a leading Catholic scholar decided to turn down the Laetare Medal, the university's most prestigious award, citing the decision to invite Obama to commencement.

It was the first time the medal has not been awarded since 1883.

"It is clear that Notre Dame didn't understand what it means to be Catholic when they issued this invitation," said Cardinal Francis George of Chicago.

The controversy comes at a time when recent polls indicate that Americans' views of abortion rights are changing.

A Gallup survey released last week found that 51 per cent of respondents consider themselves "pro-life," while 42 per cent consider themselves to be "pro-choice."

The results marked the first time a Gallup poll has found that the majority of adults consider themselves "pro-life" since the agency first asked the question in 1995.

Gallup researchers say it is mostly Republicans and right-leaning independents who are responsible for the rise in opposition to abortion rights.

The debate will likely continue to heat up into the summer, as Obama will attempt to fill a Supreme Court vacancy when Justice David Souter retires.

Anti-abortion advocates are hoping for a like-minded judge who will seek to overturn Roe vs. Wade. Souter had opposed moves to overturn the decision.

With files from The Associated Press