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Protestors ignored safety warnings and took to the water as part of the Great Rebel Raft Regatta.
Climate campaigners had been told by HM Coastguard and Police that any water-borne assault on Kingsnorth power station would be irresponsible and illegal under a river by-law.
But shortly before 8.30am a group of protestors emerged from bushes in between the Strand and the Riverside Country Park, Gillingham and carried four rafts across the mudflats to the river.
Minutes later another two rafts were launched further down stream.
At the same time three more rafts, each carrying about five people, launched from a creek behind Hoo Island.
However they were quickly intercepted by police launches, with help from the police helicopter and patrol boats from Medway Port Authority. Three people reached Kingsnorth jetty, but were immediately arrested.
Kent Police say 123 people were rescued or taken off the river, including one found swimming.
Some protestors were taken aboard and their rafts towed ashore. It is believed that police are landing the water-borne demonstrators at Upnor.
Small groups of protestors have continued to launch through out the morning. But each attempt is being foiled by a coordinated effort by the police and Medway Port Authority.