Greenpeace activists Monday took over the site in a military zone south-west of Prague where the United States would like to place its missile-defence radar, the group said.
About 20 activists scaled the hill without a permit to enter the Brdy military zone in protest of US plans to erect a radar base there, spokeswoman Lenka Borakova said, according to the dpa.
"Erecting bases for the US missile shield in central Europe will lead to instability," she said.
Speaking by telephone from the spot, she said five Greenpeace climbers set up camp in treetops, hanging a 15-by-15-metre target- shaped banner there.
The military asked the protesters to leave the site by 2 pm (1200 GMT). "We do not intend to obey," the activist said.
The Czech Defence Ministry declined to disclose whether it plans to have the activists removed from the site.
"All options are open. It depends on the attitude of the trespassers," ministry spokesman Andrej Cirtek said.
The activists committed a misdemeanour punishable by a fine by staging a political protest on military premises.
Polls have repeatedly shown that more than six out of 10 Czechs oppose hosting the US facility.
Despite adverse public opinion the Czech and US governments are close to reaching a deal on placing the base on Czech territory.
The deal requires approval in the parliament where the project lacks clear support.
Washington is also negotiating to accompany the radar with 10 interceptor missiles in Poland. The plan to place US installations in former Soviet satellites has irked Russia.