A ban on paying for sex will drive prostitution underground and make it more dangerous, sex workers have claimed.
A motion to criminalise the purchase of sex was voted through overwhelmingly by MLAs following a lengthy debate at Stormont on Monday.
Supporters said Clause Six in DUP peer Lord Morrow's Human Trafficking and Exploitation Bill tackled the main driver for human trafficking, but opponents maintained it would be too difficult to police.
While the legislation has yet to become law, the PSNI gave it "qualified support". But campaign group Ugly Mugs, on behalf of Northern Ireland sex workers, said the Bill would put people in the industry at greater risk.
It added: "We ask the Assembly to reconsider this law and look at the evidence. It will not reduce trafficking and it will make working conditions more unsafe."
But the Evangelical Alliance in Northern Ireland praised the Bill as a "step in the right direction".
Spokesman David Smyth said: "There are an estimated 29.8 million people in slavery today - more than were transported during the slave trade. Our hope is that this law will lead to Northern Ireland becoming a world leader in tackling trafficking."
Former senior police officer Jim Gamble, who previously headed up the Child Exploitation and Online Protection centre, said he believed the PSNI would struggle to enforce the law.
"I think the police recognise they have to concentrate on the organised crime element," he told this newspaper.
"I think they recognise the need to engage those who are trafficking and forcing victims to participate in the industry.
"That in itself is a huge and difficult task. Driving the rest of the clients and the service providers underground isn't going to make that any easier for them.
"I spoke to Lord Morrow very early doors when he was talking about this idea. Who wouldn't support it in principle? The problem will be in practice. It's well intentioned but, perhaps in the long-run, ill-thought-through when it comes to the practical application."
When asked for the force's stance on the move, a PSNI spokesman said: "The Police Service of Northern Ireland has provided qualified support for the Human Trafficking and Exploitation Bill.
"This is based on the positive deterrent value of Clause Six, balanced by the different enforcement framework that operates in Northern Ireland."
After the Assembly voted by 81 to 10 in favour of making it a crime to pay for sex, Lord Morrow insisted the ban was enforceable.
He added: "A huge amount of effort went into my Bill, even before it was introduced to the Assembly in June last year. Since then it has undergone detailed scrutiny. That highlighted areas which required amendment and the addition of extra clauses.
"The Bill debated yesterday was an enhanced version of what I originally brought forward, and I thank everyone who contributed to that process.
"This is not the final stage of its passage through the legislative process. However, I now look forward to seeing it passed into law next year. It is legislation which can make a major difference to some of the most vulnerable people in our society.
"I am delighted that the Assembly voted so overwhelmingly in support of that principle, and when made aware of the problem did not simply choose to look the other way."
Northern Ireland will become the first part of the UK to back a paying-for-sex crackdown thanks to the Assembly vote. The proposal was among a number of clauses contained in a Bill aimed at amending laws on trafficking and prostitution. Paid-for consensual sex is legal in Northern Ireland, though activities such as kerb-crawling, brothel-keeping and pimping are illegal.