NATO leaders arrived in Brussels amid tight security for a summit Thursday dominated by the terrorist attack in Manchester, England.
It is the first encounter of many of the leaders with U.S. President Donald Trump, who during his campaign called the alliance "obsolete" for not effectively countering terrorism.
Recent terror attacks in Europe mean the U.S. leader may get the support he needs to strengthen the alliance's anti-terrorist efforts.
Thousands of leftist demonstrators marched in central Brussels Wednesday, protesting against both NATO and Trump. Some carried picket signs and banners with obscenities insulting the U.S. leader.
Some said they actually welcomed Trump's earlier criticism of NATO, but are now disappointed that he is engaging the alliance.
Trump arrived in Brussels at a time when Europe is extremely tense following the terrorist attack in Manchester.
The Belgian government deployed about 4,000 police, some heavily armed, and soldiers in Brussels ahead of the summit. Soldiers were seen patrolling streets, hotel lobbies and the city's underground metro system.