Dozens of activists on board nine boats, including Greenpeace's famous Rainbow Warrior, landed at the site in Kent in eastern England, displaying images of climate disasters at the existing plant in an attempt to stop the construction.
"Coal is the most climate-wrecking form of electricity generation yet invented," Ben Stewart from Greenpace was quoted as saying at the site.
"A new Kingsnorth would... destroy any chance we have of persuading China and India to stop building coal plants."
While countries around the world try to slash greenhouse gas emissions to alleviate climate change, developing countries such as China and India are building a new fleet of coal plants.
E.ON, headquartered in Germany, wants to replace its existing 1,940 megawatt coal-fired-plant with what it calls two new cleaner coal units.
Britain's new Energy and Climate Change Minister, Ed Miliband, is expected to decide within the next few months whether to allow the plant.
In restating its position, E.ON said in a statement: "We believe cleaner coal has an important role to play as part of a diverse range of power generation in UK."