Those caught breaking the new law, which passed last year in the Northern Ireland Assembly by 81 votes to 10, face up to a year in prison or a £1,000 fine ($1,500 or 1,400 euros).
It will now not be illegal to solicit for sex in a public place, following the example of a Swedish law introduced in 1999.
The new law is part of the Human Trafficking and Exploitation Act 2015, and it is hoped that it will help deter traffickers.
But critics say it will drive prostitution underground, will be difficult to enforce and infringe the rights of sex workers.
Laura Lee, a sex worker and law graduate, said she will launch a legal bid at Belfast’s High Court to overturn the law, citing European human rights legislation.
“I am doing this because I believe that when two consenting adults have sex behind closed doors and if money changes hands then that is none of the state’s business,” she told the Guardian.
“The law they have introduced has nothing to do with people being trafficked but simply on their, the (ruling party) DUP’s, moral abhorrence of paid sex.”
Northern Ireland has a devolved assembly, which has the power to legislate in a wide range of areas that are not explicitly the responsibility of Westminster.
Gay marriage and abortion are outlawed in Northern Ireland but are legal in the rest of Britain. -AFP