Following a march with 500 monks and lay supporters in Yangon on January 16, U Wirathu gave the crowd a rousing speech in which he decried the United Nations’ interference in Myanmar.
“Mrs Yanghee Lee, you may be a big shot in the UN, but in our country, you are just like a prostitute,” he said, shown in a video published by the media.
“If you like, you could give your ass to kalars [a racist term] but we will not give our country to kalars,” he told the crowd.
“[Mr] Ban Ki-moon, you are an Asian like us, don’t be arrogant with us as we are both Asians,” he said, referring to the UN secretary-general.
U Wirathu told AFP that monks had decided to protest against the UN "as they are trying to interfere in our country's internal affairs."
The march came as the UN Special Rapporteur for human rights in Myanmar Ms Lee finished her second fact-finding visit to the country, during which she visited Rakhine State and Shan State.
Myanmar is still suffering from the fallout from the communal conflict in Rakhine State in 2012 that left about 200 people dead and around 140,000 trapped in squalid displacement camps, mainly the Rohingya, many of whom have fled the country in their tens of thousands heading for Malaysia and beyond.
"These people are Bengalis not Rohingyas," U Wirathu told AFP, using a term seen as disparaging to the Rohingya, many of whom claim long ancestry in Myanmar.
He claims the people are illegal Muslim immigrants from Bangladesh, whom he says pose a threat to the Buddhist majority in Myanmar.
"I don't accept them because they are dangerous to our country, not because I want them to suffer," he added.
The Myanmar government does not recognize the term Rohingya or recognize the people as citizens. Nay Pyi Taw has started a programme in Rakhine State to deal with this minority that offers citizenship only to those who are willing to label themselves as Bengali.
The UN has so far not responded to the insult.