The Washington area's public transit agency is vowing to "vigorously defend" its advertising guidelines after the American Civil Liberties Union on Wednesday filed a lawsuit against it for refusing to display advertisements promoting Milo Yiannopoulos' book.

"In 2015 WMATA's board of directors changed its advertising forum to a non-public forum and adopted commercial advertising guidelines that prohibit issue-oriented ads, including political, religious and advocacy ads," the Washington Metro Area Transit Authority said. "WMATA intends to vigorously defend its commercial advertising guidelines, which are reasonable and viewpoint-neutral."

The ACLU contends that parts of the policy violate the First Amendment by discriminating against particular ads and advertisers deemed controversial by WMATA officials.

The policy was instituted in 2015 after a set of anti-Muslim advertisements sparked controversy. The guidelines ban ads "intended to influence members of the public regarding an issue on which there are varying opinions," ads that "support or oppose an industry position or industry goal without any direct commercial benefit to the advertiser," and ads "intended to influence public policy," among others.

The ACLU also filed on behalf of abortion provider Carafem, which specializes in family planning and abortion care, and the People for the Ethical Treatment of Animals.

"This case highlights the consequences of the government's attempt to suppress all controversial speech on public transit property," Arthur Spitzer, legal director of the ACLU-DC and lead counsel in the case, said Wednesday. "The First Amendment protects the speech of everyone from discriminatory government censorship, whether you agree with the message or not."

The ACLU sees inconsistencies in how the policy has been applied. The group argues that WMATA has accepted a wide variety of advertisements conveying controversial viewpoints, pointing to its rejection of PETA's ad for a vegan diet, while approving an ad from a restaurant with the caption. "PORKADISE FOUND." The group also noted that ads for a movie that showed four women ogling a male stripper were displayed at the same time it tore down ads for Yiannopoulos' book.

"We did not take this decision lightly. We understand the pain caused by Mr. Yiannopoulos' views. We also understand the importance of the principles we seek to defend," James Esseks, the ACLU's LGBT and HIV Project director said.

The suit asked the court to declare portions of the WMATA advertising guidelines unconstitutional because they violate free speech rights and are unconstitutionally vague. A motion also has been filed on behalf of Milo Worldwide LLC, seeking immediate financial restitution for the ongoing loss of revenue from book sales caused by what it says is the wrongful removal of advertisements for his book.