"The scene was about 100 yards (90 metres) away from the preaching event," said an officer from police headquarters in the capital Naypyidaw who asked not to be named.
Police said five people -- a Buddhist child novice monk, three women and one man -- were slightly injured in the blast, which occurred in a residential area of Myanmar's second largest city on Sunday evening.
"We do not know the cause of the blast yet and are still investigating. But we think it could be from a homemade device," the officer told AFP.
The cleric Wirathu confirmed the incident and blamed "the minority" linked to an article in Time magazine, which highlighted his anti-Muslim sermons as a key factor in inciting a wave of deadly religious violence this year.
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Several episodes of unrest -- mainly targeting Muslims -- have exposed deep rifts in the Buddhist-majority country and cast a shadow over widely praised political reforms since military rule ended two years ago.
Myanmar in June banned the controversial Time magazine cover story on Buddhist-Muslim unrest, which featured a picture of Wirathu and the caption 'The Face of Buddhist Terror'.
In an article on his Facebook page entitled "This is the beginning of the cultural acts of the minority loved by Hannah Beech", Wirathu said the blast had injured audience members.
The monk has been the focus of scrutiny after emerging at the forefront of a nationalistic group calling for the boycott of Muslim businesses by Buddhists. He has recently campaigned for restrictions on marriages between Buddhist women and men from other faiths.
Communal unrest last year in the western state of Rakhine left about 200 people dead and up to 140,000 displaced, mainly Rohingya Muslims.
Some robed monks have taken part in the clashes.