NATO Secretary General Jaap de Hoop Scheffer suffered a small blood clot and fainted after opening a NATO exhibit near the Belgian royal palace on Tuesday.
Belgian Prime Minister Herman Van Rompuy and Defence Minister Pieter De Crem helped try to revive him on a park bench, The Associated Press reported.
NATO spokesman James Appathurai said the outgoing alliance chief "had a small blood clot" that was removed during a hospital procedure. Appathurai said de Hoop Scheffer is resting and will stay in the hospital for observation for three days.
He had been taking part in Belgian National Day festivities in Brussels.
Appathurai said he had been extremely tired in recent days due to a heavy schedule. De Hoop Scheffer is slated to step down as NATO secretary-general next week. Former Danish prime minister Anders Fogh will take over the post.
Earlier, de Hoop Scheffer had given a speech in London, where he urged Western nations not to give up on Afghanistan despite the fact July has become the bloodiest month for troops since the U.S.-led invasion of Afghanistan.
"Withdrawing now would mean Afghanistan again to the Taliban. It would mean a destabilization of the region. And it would mean an export of terrorism," de Hoop Scheffer said.
His pep talk comes as 55 Western soldiers, including four Canadians, were killed in July. A fifth Canadian soldier died this month from wounds he suffered in June. Eighteen British soldiers also died in July.
Like Canada, Britain has complained that some NATO countries have left the fighting to a small number of nations.
De Hoop Scheffer said he regrets more countries haven't agreed to lift rules that prevent their soldiers from fighting but tried to play down any divisions within the alliance.
"I think all NATO allies are pulling their weight. There are 14 nations fighting in the south. Other nations also take casualties. What I would like to underline is that we are there in a team."