Cameron Spencer via Getty Images Protesters are taking to Newcastle harbour in a bid to stop coal ships.

A national day of action against fossil fuel is underway, with hundreds of protesters at Newcastle calling on the federal government to make the switch to renewable energy.

The organiser of the Break Free event, 350.org, said protesters would kayak into the port of Newcastle in an attempt to stop coal ships from moving through the area.

A flotilla of kayaks is due to occupy the water from 11am. Green's Leader Richard Di Natale is expected to make an address at Horseshoe Beach around midday.

The group said one protester had been arrested after occupying the anchor chain of the coal ship Tangerine Island.

NSW police said 4 people had been arrested so far and that an inflatable boat had been seized.