Anti-nuclear protesters blocked access to Nato headquarters in Brussels today with a truck bearing a replica missile.
Three Greenpeace activists scaled a roof inside the compound in a security breach before a meeting of alliance chiefs.
Greenpeace said 24 of its members were arrested as Belgian police broke up the protest, which was over before US Defence Secretary Donald Rumsfeld and other Nato defence ministers arrived for the talks in Brussels.
The truck, carrying a replica of a US bomb, was removed and its driver taken away with the steering wheel of the vehicle chained to his body.
"It was an unannounced demonstration. It was a very dangerous thing to do and could have had regrettable consequences given that we are on an enhanced state of alert," said a Nato official. He said security guards had used tear gas to disperse the demonstration, which ended calmly after around 30 minutes.
Greenpeace said it was protesting against what it said were the 480 US nuclear weapons still based in six countries in Europe.
Nato was due to hold a meeting of a nuclear policy committee during the talks in Brussels.