US President Barack Obama has been heckled as he addresses students at the country's largest Catholic university, amid a row over abortion rights, BBC reported.
Anti-abortion activists have protested against the president's visit to Notre Dame, in Indiana.
Protests and vigils have been held on and around campus, with several people arrested before Mr Obama's address.
Mr Obama, who received an honorary degree, defends the right to abortion but says the procedure should be rare.
After entering the White House in January the president also quickly moved to reverse a Bush-era ban on embryonic stem-cell research, angering many social conservatives.
There are approximately 60 million Roman Catholics in the US, with almost half of them supporting abortion rights, research suggests. Some 42% believe abortion should be illegal.
President Obama, wearing the blue robes of Notre Dame, was welcomed onto the stage with a lengthy ovation from students and staff.
Support for the president's visit is strong among those in the audience, who watched him receive an honorary degree before delivering the commencement, or graduation, address.
He was heckled soon after beginning his speech, prompting a majority of those in the audience to cheer the president, who asked for calm and told listening graduates: "We don't do things the easy way."
The decision to invite Mr Obama to Notre Dame, the largest Catholic university in the US, has been fiercely criticised.
Many activists and Church leaders have directed their ire at university authorities they accuse of betraying Catholic principles.
"It is clear that Notre Dame didn't understand what it means to be Catholic when they issued this invitation," said Cardinal Francis George, president of the US Conference of Catholic Bishops.
Bishop John D'Arcy, of the diocese where Notre Dame is located, said he would boycott the graduation for the first time in his 24 years as bishop, the Chicago Tribune reported.
And conservative critics have described the awarding of an honorary degree as "inappropriate".
A strong majority of students are reported to be in favour of the president's visit, but a petition containing some 360,000 names was delivered to the university as a mark of protest.
Overnight, students opposed to abortion rights attended an all-night prayer vigil to protest against Mr Obama's visit, and an estimated 200 people attended a prayer session at Alumni Hall Chapel.
More than 100 protesters gathered and 23 marched onto the campus on Saturday, the Associated Press reported. Police arrested 19 for trespassing, with four also charged with resisting police.
A university spokesman said none of those arrested were students, AP added.
Mr Obama's senior advisor, David Axelrod, said on Sunday the president may touch on the abortion issue during his speech, but said he would not dwell on divisive issues.
And Professor Scott Applebey of Notre Dame's history department said Catholics were "accustomed to making their own decisions".
"That's one of the results... of living in a democracy like ours," he told the Chicage Tribune.