The West Australian premier, Roger Cook, has urged the community to condemn the emergence of “dog whistling” and the “language of division” in mainstream politics after a 20-year-old man was charged with preparing a terrorist attack.

Jayson Joseph Michaels, from Bindoon, appeared at the Perth magistrates court on Friday, charged with acting in preparation for a terrorist act, possessing a prohibited weapon, two firearms offences and using a carriage service to menace or harass.

The WA premier joined Muslim leaders and the WA police commissioner at a Perth mosque on Saturday, where Cook urged political leaders to “lower the temperature” of debate, especially over immigration.

After searching a home at Bindoon, about 75km north of Perth, police said they found “a manifesto-style document outlining plans for a nationalist and racist ideologically motivated act of violent extremism involving mass casualties”.

Police allege Michaels was motivated by white supremacy.

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Cook said he wanted to echo the prime minister, Anthony Albanese, who said after Michaels’ arrest that “political leaders have a particular responsibility not to fan the flames of bigotry”.

“Stop the language of division, which you see entering the mainstream political debate today – dog whistling under the guise of immigration policy,” Cook said.

“People who would demonise whole sections of our community. It is not acceptable, and we must all, as a community and as one, condemn it and stand up to it.”

The One Nation leader, Pauline Hanson, was widely criticised earlier this month for suggesting there were no good Muslims.

On Saturday, the Imam of the Perth mosque, Mohammed Shakib, said “divisive rhetoric has to stop” in politics.

“How on earth can you have a certain politician that demonises an entire faith group? 1.8bn [people]. That’s a quarter of humanity,” he said.

“I voiced my concerns to the premier that had anyone of us said something similar, that I don’t think there are any good Christians, for example, no one of any sound mind would say that.

“Imagine the legal repercussions. Imagine the outrage. But yet, Islamophobia has become so normalised, it’s become mainstream.”

Asked by a journalist if he had been referring to Hanson’s comments, Shakib said: “I think it’s not rocket science, I was referring to this.”

Cook said terrorism was “an act of hatred, of division … making us afraid to be who we are” and said his government would do “everything we can to ensure that we stamp it out in WA”.

The WA police commissioner, Col Blanch, on Saturday said white supremacy was not “prevalent” in the community, but there was a “very, very small minority” of people who sought to do harm.

Police have alleged Michaels was involved in a group chat with other Australians and people overseas on the encrypted messaging service Telegram.

Blanch said counter-terrorism police were continuing to go through the messages, but, so far, Michaels appeared to be the only West Australian in the group of white supremacists “talking about their hate for other people in our community”.

The commissioner said Michaels was not known to police and that his family was cooperating with the investigation.

Asked if he personally believed young men could “radicalise themselves” online, Blanch said: “Young, lonely men need care and love and support and friends and purpose in life, or they find people online to exploit their feelings, and they go down the wrong pathway.”

Nevertheless, he said police were looking for anyone who had potentially influenced Michaels into his allegedly extremist beliefs.

Michaels, who was refused bail, is next due to appear in court on 23 March.