Flags at half-mast have been a common sight in Brussels in recent years.
This time in memory of those killed in Manchester.
It follows other terror attacks on European soil in Paris and the Belgian capital itself.
“I would like to convey my deepest sympathies to Prime Minister (Theresa) May and to the British people,” said Jean-Claude Juncker, the European Commission president , in a statement.
“Today we mourn with you. Tomorrow we will work side by side with you to fight back against those who seek to destroy our way of life. They underestimate ours and your resilience – these cowardly attacks will only strengthen our commitment to work together to defeat the perpetrators of such vile acts.”
The former Luxembourg prime minister’s words were echoed by his EU foreign policy chief Federica Mogherini.
“This is a very day for Europe and I would to start by expressing my personal condolences and on the name of the EU institutions to all the loved ones of the victims of the terrorist attack in Manchester and to extend our condolences to all the citizens of the UK and Prime minister,” she said.
The Manchester bombing is the worst terror attack in the UK since the London attacks of July 2005 when 52 people were killed