The helicopters enforcing a no-fly zone around the summit in the Baltic resort of Heiligendamm swiftly circled the balloon, deliberately creating turbulence and forcing it to land in a field some 20 km (12 miles) from the venue.

"We were hoping to fly in a petition to the G8 leaders demanding binding targets for CO2 cuts," aid Svenja Koch, a spokeswoman for the environmental activists.

"But unfortunately the police forced us down. Their helicopters caused too much turbulence and it wasn't possible to continue."

On Friday afternoon, a group of naked protesters jumped into the Baltic Sea in front of the media centre in Kuehlungsborn just west of Heiligendamm. Several were detained by police as they swam towards a security fence.

"Make capitalism history," was written on their backs as they jumped in. Nude bathing is popular in eastern Germany. Nudists calling themselves "the naked truth" have surfaced at rallies around the G8 summit.

"It was a great way to finish of a great week," said one young German woman after emerging from the sea. "We've got the power -- and we showed it to those who think they're powerful."

In the nearby city of Rostock, police said two groups of about 5,000 marched in a peaceful rally after ending their blockades at Heiligendamm.

BACK DOWN TO EARTH

The blue and white Greenpeace hot air balloon had a giant yellow banner with the slogan "G8 Act Now" hanging below it, with the word "Failed" stamped across it. It was in the air for around 15 minutes before it was forced down.

"The hot air balloon was spotted immediately," said police spokesman Manfred Luetjann. "The air space is closed. They did not get very far. The two people on board were taken into custody."

On Thursday, police boats rammed two inflatable Greenpeace speed boats that breached the security zone in the Baltic Sea in front of Heiligendamm, tipping activists into the water and injuring three of them.

Land access to the G8 summit venue, a luxury hotel complex in the small seaside town of Heiligendamm, was blocked for a third day on Friday as thousands of anti-globalisation protesters camped out on several nearby roads.

G8 leaders at the summit agreed on Thursday to pursue "substantial" cuts in greenhouse gases to combat global warming, but stopped short of committing to firm reduction targets.