The world's biggest heavy metal music festival starting in northern Germany on Wednesday was forced to cap attendance after heavy rains turned its outdoor venue into a muddy quagmire.

Organisers of the annual Wacken Open Air (W:O:A) bash, featuring 150 bands on eight stages over four days, said the event would go forward as planned but with only around 60 percent of the 85,000 ticket-holders after imposing an entry freeze.

In messages to "dear metalheads" on social media, the promoters said they regretted having to cap the total audience at the around 50,000 fans who were already present, calling the situation "very sad". 

"We tried everything but unfortunately we are unable to allow anyone else onto the Holy Ground," they said, referring to the sprawling rural venue.

"There is no other way," they added, calling it the "first time we've taken this decision in the history of the W:O:A."

Festival co-founder Holger Huebner said in a video message that ticket-holders who were refused entry would receive refunds. 

"We're on it, please have a little patience," he said.

Launched in 1989, Wacken is billed as the world's largest heavy metal gathering and regularly draws the scene's biggest bands, with headbangers travelling from across Germany and abroad to take part.

Britain's Iron Maiden, US thrash metal act Megadeth and folk-punk group Dropkick Murphys are among the headliners this year.

On Wednesday, some of the ashes of Motorhead lead singer Ian "Lemmy" Kilmister, who died in 2015, were brought to the W:O:A fairground in a tribute ceremony before they go on display in an urn. The band played Wacken eight times.

- 'Incredibly lucky' -

Although often a muddy affair, Wacken is facing particularly severe accessibility problems due to a steady downpour.

Weather forecasts indicated the torrential rain in northern Germany would continue, with hail and squalls possible, leading organisers to take the precautionary measures without halting the event entirely.

The mood at the showgrounds remained festive as the event prepared to kick off, with black-clad festival-goers making their way ankle-deep in muck. Fans using wheeled walkers and even a wheelchair proved undeterred.

Lene Fuchs, who said she arrived in Wacken from the southern town of Hohenheim on Monday, noted that organisers were doing their best under the circumstances.

"We are at the camping site and got incredibly lucky," she wrote on Facebook. "The W:O:A team is doing an amazing job!"

However festival-goer Dirk Liberkowski urged the promoters to call off the event, fearing a disaster if there were an emergency at the venue.

"Cancellation is the only right decision! All the paths are impassable! At night it's really dangerous, especially for those who have been drinking." 

dlc/mfp/jmm