Former British prime minister Tony Blair has been heckled by a protester threatening to perform a citizen's arrest as he prepared to make a speech on religion and globalisation at the University of Hong Kong on Thursday.

Mr Blair was in the city for the inaugural speech of the Tony Blair Faith Foundation, which aims to promote understanding between religions.

But almost as soon as he took to the rostrum, activist Tom Grundy walked down the steps of the lecture theatre, claiming he was arresting Mr Blair on charges of breaking international law and the Geneva Convention in relation to the invasion of Iraq in 2003.

As Mr Grundy approached the podium at which Blair was speaking, he was himself threatened with police action and left the auditorium peacefully.

Outside, Grundy told reporters: "I attempted a citizen's arrest against Tony Blair today because back in 2009 he admitted he would have gone to war with Iraq regardless of the [Weapons of Mass Destruction], and that means it's in defiance of the Nuremburg Principles, the UN Charter, the Geneva Convention, and a pending International Criminal Court, a Rome Statute."

"He misled the British people, he's caused the deaths of 100,000 people at least, and there was some obligation on Hong Kong police to actually arrest him," he added.

Mr Blair was clearly irritated by the interruption, which comes just weeks after a similar incident as he gave evidence at the Leveson Inquiry into press ethics.

"That's democracy for you," Mr Blair said as Grundy walked out, adding that he was used to it.