NEW YORK — A severed pig’s head was left outside a mosque in Philadelphia. An Islamic center in Florida was defaced. A Sikh temple in California was vandalized by someone who mistook it for a mosque and left graffiti that included a profane reference to the Islamic State group.

Advocacy groups believe there has been a spike in anti-Muslim incidents across the United States in recent weeks that can be linked to last week’s mass shooting in California and the inflammatory rhetoric of Donald Trump and other Republican presidential candidates. And they say that Muslims are fearful the backlash could lead to further harassment and violence.

“The spike began with the Paris attacks and has intensified with what happened in San Bernardino and now with what Donald Trump is proposing,” Ibrahim Hooper, lead spokesman for the Council on American-Islamic Relations, said Wednesday. “I have never seen such fear and apprehension in the Muslim community, even after 9/11.”

The FBI, which keeps statistics on hate crimes committed nationwide, said data about 2015 will not be available until next year. But the Anti-Defamation League said it has tracked more than three dozen incidents since the Nov. 13 terror attacks in Paris that left 130 dead.

“We’re talking at least three dozen that we’re aware of and I’m sure there are many more incidents that haven’t been reported,” said Oren Segal, the director of the ADL’s Center on Extremism.

“With legit terror attacks and the public discourse about them, it has created an atmosphere ripe for these types of stereotypes and incidents,” Segal said. “People are exploiting them.”