Assistant Chief Constable Alice Scott from Suffolk Police said the force would have visible presence in the Shrubland Hall area in the coming days.
She said 15,000 attendees had “safely departed” from the event.
Ruman Muhith, a Labour member of Ipswich Borough Council, attended on Friday and Saturday and said attendees had “conducted themselves with dignity and respect”.
Prime Minister Sir Keir Starmer said in a post on X: “Shocking news that Counter Terrorism Police are investigating a credible, serious threat targeted at an Islamic event in Suffolk.
“Thanks to the quick action taken by police and the organisers, nobody was hurt.
“My message is clear: I will not tolerate any attacks on our Muslim communities, or any form of anti-Muslim hatred.”