Four boats have been intercepted by police and coastguards on the River Medway near Kingsnorth power station.
The vessels carrying climate change protesters were stopped at Upnor which is just upstream of the Kent plant.
Officers in riot gear and about 15 police vans are stationed in front of the power station on the Hoo peninsula.
About 100 activists opposing the firm's plans to build new coal-fired units have marched to the plant gates. They say they want to shut it down.
Some protesters appeared to have breached a fence near the power station on Saturday morning.
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About 1,500 people have been stationed at the Climate Camp - which is one mile from the power station - over the past week.
Four demonstrators trying to launch a boat onto the River Medway on Saturday were arrested by Kent Police.
Officers said 13 people were held on Friday. Five were charged with obstruction, one with possession of a bladed article and one with a public order offence.
It takes the total number of people arrested since the protest started to more than 50.
Regatta banned
An E.On spokesman urged activists to keep outside the gates to prevent the possibility of serious injury.
E.On spokesman Jonathan Smith said: "If you look at a power station clearly there are many many areas where there's large scale machinery working, they are very tall structures.
"If you do not know what you are doing you could really very seriously injure yourself at a power station and we do not want that to happen."
The Medway Port Authority harbour master announced on Thursday that he would not give permission to any procession or regatta to take place under river byelaws because of safety concerns.
In July E.On was granted an injunction to prevent campaigners getting access to a jetty on the River Medway where fuel is delivered to the power station.
Prtotester Kevin Smith told the BBC they were very determined to go ahead with the protest.
"We're very concerned about the long-term sustainability of the planet," he said.
"Obviously safety is paramount in our preparations but E.On really can't go ahead with this. It's going to take us to the edge of an ecological catastrophe."
Cleaner coal
Camp for Climate Action spokeswoman Jessica Glynn added: "The last thing we want to do is injure ourselves or anyone else.
"Nobody is allowed to participate if they have not had full health and safety training and are not wearing life jackets."
E.On plans to build two new cleaner coal units on the site, which it claims will be 20% cleaner.
Medway Council has approved the scheme, which would be the first coal-fired power station to be built in the UK for 24 years, and the final decision will be made by the government.
If approved, E.On said the power station would be operational by 2012 and would provide energy for 1.5 million homes.
(BBC)
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