Scores of police in full riot gear were at the RAF Fairford airbase on Saturday afternoon, waiting for a peace march to pass.
Around 2,000 protesters gathered outside the base where 14 US air force B-52 bombers are positioned.
Three of the long-range bombers were seen flying out of the Gloucestershire base on Friday at midday, and were thought to have been involved in the evening aerial onslaught on Baghdad.
Campaigners were asked to bring wreaths to lay at the base for "the death of our democracy and for all the victims of the biggest preventable tragedy in history".
A Gloucestershire police spokesman said they had expected between 3,000 and 5,000 people to attend the protest.
It was the largest operation the force had ever mounted, he said.
On Friday, a Greenpeace hot air balloon dropped anti-war leaflets over the base.
The balloon, with the words "No War" written on the side, dropped around 500 leaflets on the base.
A Greenpeace spokesperson said the action was not about disrupting the workings of the base.
Eight B-52s returned safely to the base early on Saturday following a massive assault on Baghdad.
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