The patriotic celebration took place on Saturday night at London’s Royal Albert Hall. Traditional hit Rule Britannia was sung by Stuart Skelton, an Australian opera star.
However anti-Brexit campaigners targeted the event, handing EU flags from balconies and waving them in the hall.
Many also wore blue and gold EU hat berets, which were popular on anti-Brexit protests after the 2016 referendum.
One Europhile group, the ‘EU Flag Mafia’, said it handed out 10,000 EU flags outside the Proms “in support of Gigging British Musicians in trouble due to a failed Brexit”.
Some artists also demonstrated outside the Royal Albert Hall before the event, claiming Brexit had made it harder for them to tour the EU.
The Proms has been targeted by anti-Brexit activists before, with Nigel Farage condemning EU flag wavers in 2017 for “trying to make it all about them”.
However, EU flag-wavers came in for stringent criticism on social media.
One user tweeted: “Go away with your EU flags and hats.
“Would it kill you to celebrate with the Union Jack for one night? Be patriotic, this is the last night of the Proms.”
Another added: “I really do wonder about some folks.
“Last night of the proms, Rule Britannia, Land of Hope & Glory.
“There they are EU Berets atop their bobbling heads, waving their little flags – I mean has anyone told them we are out of the EU and it’s all over.”
A third posted: “Last night of the Proms.
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“Why are idiots wearing EU hats, waving EU flags, but singing Rule Britannia? What hypocrites.”
A fourth said: “Idiot of the week awards goes to all those Remainers with EU flags and hats at the last night of the Proms.
“Five years on and still can’t accept the result of the referendum.”
One more commented: “Last Night of the Proms Multiple……the EU flags & berets are out in force again but it’s hilarious watching them sing all our patriotic songs. Clueless doesn’t even begin to cover it.”
A tribute was paid at the Proms to the victims of the 9/11 terrorist attacks, 20 years after they took place.
Conductor Sakari Oramo said the occasion showed the “healing power of music”.
He added: “Due to the worldwide Covid pandemic, live music-making was stopped across the world for many months – and in some places it still is.
“Music that is core to human expression went silent.
“The bond between audiences and performers was broken.”
Britain voted to leave the EU in June 2016, by 52 percent of the vote to 48 percent.
Departure finally took place in January 2020, following a number of delays caused by the opposition in Parliament.