Advocates have long argued that a loophole in state law permitting indoor prostitution has allowed the sex slave trade to flourish, feeding brothels thinly disguised as massage parlors.
Police believe many of the women who work at the spas are brought from overseas and held against their will.
The bill would set mandatory minimum prison sentences for anyone convicted of trafficking minors for the purpose of sex and bring state law into line with federal penalties for human trafficking.
It eliminates the need for minors to have been coerced or forced into sexual slavery to garner a conviction.
Legislators also want training for police to recognize victims and a checklist they could use for calls involving runaways and prostitution, tools advocates say are necessary because there has never been a successful prosecution of a human trafficking case in the state.
State Police Colonel Brendan Doherty said before the start of the rally that he was working with Attorney General Patrick Lynch's office and legislators to tweak some details of the bill.
But he called human trafficking "despicable" and "untenable" and said he supported the bill.
Critics say previous attempts to tighten state law unfairly punished victims instead of pimps and captors. They don't think this bill would be any different.
Steven Brown, executive director of the Rhode Island affiliate of the American Civil Liberties Union, said he does not support the bill.
"The only significant effect it can have is to further punish women," Brown said. "It will not in any way enhance the ability of the police to go after traffickers."
Last year, loitering for indecent purposes was the most common misdemeanor charge for women at the state prison, both for those awaiting trial and those who had been sentenced, according to the Department of Corrections.
Senator Rhoda Perry, who introduced the Senate version of the bill, said at the rally that sending more people to prison for prostitution isn't the goal.
"Instead, we must focus our efforts on legislation that directly targets the real villains, the traffickers and owners of these facilities," she told the crowd.
In the House, the bill was heard by the Judiciary Committee this month. The deadline to vote on bills is tomorrow but is expected to be extended, House spokesman Larry Berman said.
