By Tom Miles

GENEVA (Reuters) -- Politicians who use derogatory language about refugees and migrants may ultimately be responsible for causing violence, racism and bigotry, U.N. Human Rights Commissioner Zeid Ra'ad al-Hussein warned on Tuesday.

"Once you classify people along lines such that they pose a threat – that these are ‘hordes’, that we are being ‘invaded’, that there are ‘swarms’ of people coming, you have started the process of dehumanizing them as we know from history," he said.

Speaking to a packed audience of students and diplomats at Geneva's Graduate Institute, Zeid said politicians may feel they were exercising their freedom of expression responsibly, but such language was not okay, even in the mildest form.

"Someone is going to pick up on this term, and down the line in some village somewhere, a family will be murdered because someone thought they had license to visit violence on this particular family because the politician stigmatized them," Zeid said.

Politicians should be free to express their wishes and their will freely, he said, but they should do so "with due regard given to history, and some responsibility".

"For the bigot, the chauvinist, the racist, it really doesn’t matter how many people turn up. Even one family that is different or foreign is enough to make an issue out of it. We have to be so careful that we don’t give ground to people like that."

Zeid did not name any countries, but British Prime Minister David Cameron attracted criticism earlier this year for referring to migrants as a "swarm".

Zeid also suggested the European Union should "absolutely forget" about economic revival if it insisted on obsessively checking every person crossing every one of its borders.

Many countries claimed to be leading a war on human trafficking, but they showed little regard for the victims and made it impossible for migrants to enter their labor markets even if they were crying out for workers, he added.

"If xenophobia and demagoguery are allowed to set the governance agenda, what we will see is more deaths and greater brutalization of society as a whole, with communities that are cleaved from one another and prospects for greater violence."

People were forced to leave countries such as Syria, Eritrea and Myanmar because of war, oppression and persecution, but the leaders of some of the world's most prosperous and privileged countries greeted them with "hate-filled rhetoric and even action", or detained them for long periods, Zeid said.

(Reporting by Tom Miles; Editing by Ken Wills)