Breitbart writer Milo Yiannopoulos has warned American universities they are being watched by US President Donald Trump following a violent protest outside his scheduled event at UC Berkeley.

The alt-right speaker, whose US university tour has prompted several violent protests that have led to his talks being cancelled, made the comments after the president threatened to pull funding from Berkeley.

In a tweet on Thursday (2 February), Trump wrote: "If UC Berkeley does not allow free speech and practices violence on innocent people with a different point of view — NO FEDERAL FUNDS?"

Yiannopoulos' talk was cancelled after a group of people demonstrated outside the university on Wednesday night (1 February), with a fire started, smoke bombs thrown and tear gas used by police to disperse the crowd.

In a text message to Business Insider, Yiannopoulos said: "American universities should be on notice.

"The president is watching. The days you could silence conservative and libertarian voices on campus and still expect to collect their tax money are coming to an end.

"I am the catalyst for this change. I'm confident Trump and his team are watching closely and will act decisively."

Prior to his scheduled appearance at the university, UC Berkely's chancellor Nicholas Dirks issued a statement in which he described Yiannopoulos as a troll but defended his right to speak freely on campus.

The statement said: "Mr. Yiannopoulos is not the first of his ilk to speak at Berkeley and he will not be the last. In our view, Mr. Yiannopoulos is a troll and provocateur who uses odious behavior in part to "entertain," but also to deflect any serious engagement with ideas.

"He has been widely and rightly condemned for engaging in hate speech directed at a wide range of groups and individuals, as well as for disparaging and ridiculing individual audience members, particularly members of the LGBTQ community."

However, the statement added that the university defended Yiannopoulos' right to free speech, explaining: "we are defending the right to free expression at an historic moment for our nation, when this right is once again of paramount importance. In this context, we cannot afford to undermine those rights, and feel a need to make a spirited defence of the principle of tolerance, even when it means we tolerate that which may appear to us as intolerant."

The Berkely protest is not the first violent demonstration outside one of Yiannopoulos' scheduled events, with a man shot during a protest outside his talk at the University of Washington.