Washington - Dozens of students at Indiana's Notre Dame University protested White House policies on Sunday by walking out of a commencement speech by Vice President Mike Pence, who criticised political correctness at American colleges.The members of the graduating class - dressed in cap and gowns together with some two thousand classmates - stood up and quietly left the school's football stadium when Pence began delivering his speech, videos posted online showed. Others cheered and some booed.Notre Dame, in the city of South Bend, is one of the country's most prominent Catholic universities.Pence, who received an honorary degree from the university, said that "far too many campuses across America have become characterised by speech codes, safe zones, tone policing, administration-sanctioned political correctness - all of which amounts to nothing less than suppression of the freedom of speech"."These all-too-common practices are destructive of learning and the pursuit of knowledge, and they are wholly outside the America tradition," he added.A religious social conservative, Pence is a former Indiana governor who was born in the state and also served as one of its representatives in Congress for 12 years.The protest comes amid a mounting controversy over what constitutes free speech at college campuses since the election of Donald Trump as president in November, with students objecting to appearances by divisive conservative figures. Some schools have cancelled events.In addition to Pence's record as governor, the protesters said they wanted to voice objections to Trump's threat to civil liberties and policies such as his attempts to ban travellers from some Muslim-majority countries."The participation and degree-conferring of VP Pence stand as an endorsement of policies and actions which directly contradict Catholic social teachings and values and target vulnerable members of the university's community," Notre Dame student Xitlaly Estrada said in a statement by the student group WeStandForND, which organised the protest.More than 100 people walked out, organisers said. Many wore rainbow pins or flags, a symbol of gay pride.Pence delivered his speech while Trump was on the first day of his first trip abroad as president. In a speech in Saudi Arabia, he called on Middle Eastern leaders to help defeat Islamist extremism.