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.
北约领导人在严密安保中抵达布鲁塞尔，参加星期四举行的峰会。峰会主题被英格兰曼彻斯特发生的恐怖袭击事件所主导。
这是很多北约国家领导人首次与川普总统会面，川普在竞选期间称北约“过时”，原因是没能有效抗击恐怖主义。
最近在欧洲发生的恐怖袭击事件意味着这位美国领导人会得到他强化这一军事联盟反恐努力所需的支持。
数以千计左翼示威者星期三游行至布鲁塞尔市中心，抗议北约和川普。一些人携带着抗议标语和旗帜，上面是侮辱这位美国领导人的下流话。
一些人表示，他们实际上欢迎川普早前对北约的批评，但是现在对他与这个军事联盟关系密切感到失望。
川普总统抵达布鲁塞尔，正值曼彻斯特发生恐怖袭击事件之后，欧洲局势极度紧张。
峰会召开前，比利时政府在首都布鲁塞尔部署了大约4千名全副武装的警力和一些士兵。可以看到军人在街道、酒店大堂和地铁等地点巡逻。