Nov 30 (Reuters) - A climate-change protest off Australia's east coast on Sunday disrupted operations at one of the country's biggest coal export ports, prompting 21 arrests, the second protest in two days to disrupt shipping at the Port of Newcastle.

Climate activist group Rising Tide, which claimed responsibility for the action, said hundreds of activists in kayaks paddled into the shipping lane of Newcastle Harbour on Sunday morning local time.

The port, 170 km (110 miles) north of the state capital Sydney, is the largest bulk shipping port on the east coast of Australia, a nation where climate change is a divisive issue.

"General cargo movements are now being aborted due to the protester disruption," a Port of Newcastle spokesperson said, adding that movements of alumina bound for Australia's largest aluminium smelter, Tomago, were "now being interrupted".

A coal ship was earlier denied entry to the port on safety concerns due to protesters in the water, the spokesperson said.

Police said in a statement that 21 people were arrested and charged on Sunday with "alleged marine-related offences" at the protest.

Greenpeace Australia Pacific said three of its activists climbed onto a coal ship near the port, stopping it from operating, as part of the "peaceful protest".

"Greenpeace, alongside Rising Tide and thousands of everyday people, are taking actions big and small this weekend," Greenpeace Australia Pacific head of climate and energy Joe Rafalowicz said in a statement.

On Saturday, a protest at Newcastle forced an inbound ship to turn back and police made 11 arrests. A similar multi-day climate action occurred last year in which 170 protesters were arrested.

Coal is one of Australia's top commodity exports, along with iron ore. Australia's government has committed to reaching net-zero greenhouse gas emissions by 2050.

(Reporting by Sam McKeith in Sydney; Editing by Michael Perry)

((Samuel.McKeith@thomsonreuters.com;))