Honolulu police worked with two federal agencies to conduct an undercover operation that led to the arrest of 16 women last week in the latest crackdown on prostitution and illegal massage parlors.

But instead of arresting the women and charging them with prostitution, the women were charged with fourth-degree sexual assault — a misdemeanor.

If convicted, the women would have to register as sex offenders and could spend up to a year in jail, while a prostitution charge carries just 30 days. Fourth-degree sexual assault is defined as sexual contact or exposing one’s genitals.

Advocates for prostitutes criticized the decision to charge the women with sexual assault.

"We’re really appalled by what law enforcement has been doing in using this unprecedented approach," said Kathryn Xian, executive director of Pacific Alliance to Stop Slavery. "Why didn’t they arrest the johns or the owners of the property?"

Honolulu Police Department spokeswoman Michelle Yu said in a statement that HPD worked with the FBI and Homeland Security Investigations to conduct the operation in response to complaints of prostitution, unlicensed cosmetology and massage business activity.

HPD also said it is working to deter sex-trafficking.

"Sex-trafficking is a very serious issue, and the HPD, FBI and HSI work closely to identify victims and to get them the services that they need," Yu said.

The arrests began April 30 with eight women, between the ages of 24 to 48, picked up at establishments near Ala Moana Center. The sting continued Friday and Saturday nights, with eight more women arrested on the same sexual assault charge also in the Ala Moana Center area.

All of the women posted bail and have dates to appear in District Court beginning at the end of the month.

FBI Special Agent Tom Simon said the FBI played a minor role in the undercover operation with one special agent accompanying police on April 30 for the purpose of determining if there was any evidence of minors being involved in the illegal sex trade. "No such evidence was uncovered," Simon said.

An HSI spokesman declined to comment because of the ongoing investigation.

Yu said details of the cases will be presented in court.

The Associated Press contributed to this report.