The rainbow arch was donated to Milton Keynes Pride Festival by My Milton Keynes.
The much-anticipated event had a turn out of over 14,000 people on Saturday, with the MKPF team saying "It was wonderful to see many people from our community and allies come together."
Following the successful day, this morning (13/9), members of the MKPF team discovered that the rainbow arch had been set on fire overnight.
Milton Keynes Pride Festival director Keith Emmett III, said: "MKPF team are extremely saddened to discover that a person or persons have deliberately broken into the PRIDE site and set fire to the beautiful rainbow arch that was donated by My Milton Keynes.
"The arch had been fenced and secured which means that the incident was undertaken on purpose.
"We had in excess of 14,000 people attend on Saturday and it was such amazing day.
"Sadly this incident shows that there is still so much work to be done within MK in regards to the Homophobia, Lesphobia, Transphobia and other forms of discrimination the LGBTQ+ community face."
My Milton Keynes said: "After a fantastic weekend of celebrations MK Pride Festival we are shocked and saddened to see that our beautiful rainbow flower arch has been destroyed by vandals.
"MyMiltonKeynes is proud to champion improvements across our city, for the benefit of the entire community. More than 14,000 people attended on Saturday and it was such an amazing day for all.
"We will not let people like this hurt us and will ALWAYS CHOOSE LOVE."
If you have any information, please report to Thames Valley Police by calling 101 and reporting reference number 432104117766.