US civil rights groups are calling for better protection of American mosques after several in California were targeted with letters threatening genocide of Muslims.

The handwritten letter was addressed to “the children of Satan” and expressed support for President-elect Donald Trump. It also called Muslims “vile and filthy people.”

“Your day of reckoning has arrived. There’s a new sheriff in town, President Donald Trump. He’s going to cleanse America and make it shine again. And, he’s going to start with you Muslims,” the letter stated.

Signed only by “Americans for a Better Way,” the letter said Mr Trump was “going to do to you Muslims what Hitler did to the jews [sic].”

CAIR is now calling for greater police protection of mosques as hate crime in the US surges in the wake of Mr Trump’s election victory.

According to the group’s executive director, Hussam Ayloush, the centres were unsure whether they wanted to go public with the story for fear of copycats or fuelling further fear in the Muslim community.

However, once they learned of another letter sent in Northern California, they wanted to inform the public and encourage police to look into the matter.

“You always want to trust that most people are good, that your neighbors have been good, and that has been our experience,” he told the LA Times.

However, he blamed Mr Trump’s “irresponsible, hateful rhetoric” for fuelling “a level of vulgarity, vile hatred and anger among many self-proclaimed Trump supporters.”

“I’m not saying Trump created racist people,” he said.

“He normalised it. While he might say he’s not responsible, and I respect that, I remind President-elect Trump that he has a responsibility to act as a president for all Americans.”

According to the Southern Poverty Law Centre, there have been at least 701 examples of hate crime since 9 November, including one woman being told: “I’ll spit on you, you dirty bitch. I can smell the Africa on you”, with growing fears anti-Muslim hate crimes could be reaching post-9/11 levels.

Anti-Muslim hate crimes have also grown since in the past year according to FBI data, with 257 anti-Islamic-motivated crimes reported in 2015, compared to 296 in 2001.