Three men have been arrested under new legislation aimed at protecting women who have been forced into prostitution.
They were held by officers at an alleged brothel in Newham, east London, a Metropolitan Police spokesman said.
The men, two aged 37 and one aged 35, were questioned on suspicion of paying for the sexual services of a prostitute subjected to force.
The 35-year-old was given a caution. A 25-year-old woman was also arrested on suspicion of managing a brothel.
The arrests were made on the day new laws came into force to deal with men who, by paying for sex, are involved in forcing women into prostitution against their will.
'Trade of abuse'
From 1 April, a man paying for sex with a prostitute who is being exploited through force, deception or threats is breaking the law - regardless of whether or not he knows she is selling her body against her will.
Under the new legislation, courts have the power to close down premises associated with certain prostitution and pornography offences.
And a new penalty for the offence of soliciting will oblige sex workers to attend meetings at which they will be helped to leave prostitution.
Det Insp Kevin Hyland said: "Men who visit brothels and pay for sex are exploiting vulnerable woman and plying a trade of abuse.
"The new legislation, enforceable from today, means that people who pay for sex in brothels can be arrested and appear before the courts, which we hope will act as a deterrent to others."