Muslims are being urged to stay away from the Sydney Royal Easter Show by a community group linked to Islamist fundamentalists.

Sydney Islamic Lectures posted a "must share ASAP" message on Facebook as thousands of people flocked to Homebush in the city's west for the opening day of the annual showcase of agricultural produce, according to Daily Mail.

"The Easter Show is on again this starting week. Many Muslims are attending without knowledge," it said.

The group, which posts links to Islamist groups and sharia law lectures, shared a YouTube video of fundamentalist Sheikh Feiz Muhammad.

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He gives weekly fiery sermons at Auburn, which is a few suburbs away from Homebush, where about 850,000 people are expected to attend the Easter Show during the next 13 days from Thursday.

The fundamentalist founder of the Ahlus Sunnah Wal Jamaah Association in Auburn told his followers Muslims should not take part in non-Muslim festivities.

"Is it part of the sharia? Are we allowed to entertain ourselves with celebrations that are built on non-Muslim concepts?," he said.

"If you believe that this is accurate, is acceptable, is their right and I agree, hence I'm celebrating, you're one of them."

In his 20-minute sermon, he said it was sinful for Muslims to take part in any festivity associated with another faith.

"If you go on the belief, 'I just want to join in and have the fun, you know, just have a night out, and enjoy myself but I don't believe in all this nonsense', that's a major sin," he said.

"It's a what? It's a major sin."

The video was produced in December 2016, telling Muslims that celebrating Christmas or New Year would be the work of the devil.

The Royal Agricultural Society of New South Wales said Muslims were "most welcome" to attend the Easter Show.

"Many of our media releases are translated for them in Arabic and uploaded on our website so we can appeal to all sorts of communities," spokesman Roger White told Daily Mail Australia.