BOULDER, Colo. -- An attorney is threatening legal action if the University of Colorado doesn't waive a $3,000 fee for student groups hosting an appearance by former Weather Underground member Bill Ayers and fired CU professor Ward Churchill.

Attorney David Lane said Thursday the fee is exorbitant and an unconstitutional restraint on free speech.

"It is our intention to proceed to federal court to obtain an injunction if CU attempts to stop this event by charging a fee to hold it," Lane said.

CU attorney Pat O'Rourke said he's reviewing Lane's demand. CU-Boulder spokesman Bronson Hilliard said it's standard to charge student groups fees to recoup security costs for large events.

He said the $3,000 figure is toward the lower end of what the school charges.

Ayers and Churchill are scheduled to speak at a March 5 event sponsored by three groups.

Ayers is a University of Illinois-Chicago professor who featured prominently in Republican efforts to thwart Barack Obama's presidential campaign. Churchill was fired on plagiarism allegations after he equated some Sept. 11 victims to a Nazi.

"The First Amendment applies equally to the rich as well as impoverished students. To charge an exorbitant fee to exercise free speech is unconstitutional. Charging $3,000 for free speech takes the 'free' part out of the equation," Lane wrote to CU's attorney Patrick O'Rourke. "My student/clients have a right to put on this event and if the university feels that extra security is necessary, that’s what we pay tax dollars for police protection to accomplish. Just as Martin Luther King could not be stopped from marching through the streets for failing to have a large insurance policy or money to pay the police, these students will not be deterred from exercising their First Amendment rights by fees charged by CU."