Police spokeswoman Mignon van der Laan told AFP the group of about 90 people had set up camp on the perimeters of the Maasvlakte building site of power company E.ON on Friday night.

"This morning, in spite of an agreement with the police, they entered the site and were therefore trespassing," she said.

About 50 of the group chained themselves to machinery, buildings and cranes on the site.

"We are arresting all of them and giving them a fine. Some are refusing to give their names and some are resisting -- we may have no choice but to take these individuals to jail," said Van der Laan.

The process would take several hours, she added.

The journalists present were also fined for trespassing.

The Greenpeace ship Rainbow Warrior was offshore with another few dozen people on board to provide moral and logistical support to the activists on the ground, spokeswoman Agnes Derooij told AFP from the ship.

Greenpeace said in a statement the action was in protest of an "unfolding climate disaster." They intended to stay there "until the coal plant is cancelled."

"The consequences for the climate from this coal plant would be so dramatic, that urgent action is needed now," said the head of Greenpeace's energy and climate campaign, Meike Baretta.

The body contends that coal is responsible for a third of global greenhouse gas emissions.

"Coal-fired power stations undermine European targets to cut emissions by 30 percent by 2020. Quitting coal is essential to a meaningful deal to save the climate."

E.ON said in a statement earlier in the year that the new power station had been approved in line with Dutch nature conservation laws.

"Its environmental performance is leading in the world and fits in with the plans of the Rotterdam/Rijnmond area to continually improve the air quality," it said.

E.ON spokesman Hans Schoenmakers told AFP from the site the activists were entitled to protest, but "what we don't like is that they enter our premises and create unsafe conditions".

"Of course, we cannot deny that carbon dioxide is emitted from power plants, but we do try to do this in as clean a way as possible.

"Electricity has to be produced, and it cannot all come from wind and solar energy."