Nineteen U.S. states have introduced bills that would curb freedom of expression and the right to protest since Donald Trump's election as president, an "alarming and undemocratic" trend, U.N. human rights investigators said on Thursday.

Concerns for free speech in the United States have risen in part because of the Republican Trump's antagonistic relations with prominent U.S. media, which he has branded "the enemy of the American people" as it has reported on policy missteps and dysfunction in his administration.

The push for stricter laws on expression has come as Trump's liberal foes have pursued public protests over his policies on issues ranging from immigration to abortion and climate change.