Prime Minister Boris Johnson skipped a planned press conference with his Luxembourg counterpart because of protests.
There were cheers as Luxembourg's PM Xavier Bettel pointed to the absent British Prime Minister's podium and challenged the UK's failure to find a solution to Irish border question.
Protesters lined the streets of Luxembourg around the site of Mr Johnson's meeting with European Commission President Jean-Claude Juncker.
Mr Johnson decided not to take part in an outdoor press conference as heckles and boos grew in volume.
Mr Bettel said: "Our people need to know what is going to happen to them in six weeks' time. They need clarity, they need certainty, and they need stability.
"You can't hold their future hostage for party political gains."