The 2012 London Olympics could become a magnet for human traffickers bringing in prostitutes and illegal workers, the government has said.
Thousands of women are forced to work in the sex trade
The Home Office has unveiled plans to combat gangs who imprison women and force them into the sex trade and push men into forced labour.
The UK will allow freed victims to stay for at least 30 days for counselling.
Police Minister Vernon Coaker said human trafficking was "a form of modern-day slavery".
The Home Office's action plan says: "There is little doubt that there will be many who will seek to prosper from the Olympics being held in London.
"Hospitality, catering and construction workers will be required. Criminal elements are expected to exploit the situation by establishing themselves in London from now on."
An influx of young male sports fans, such as happened during the 2006 World Cup in Germany, could see a rise in demand for prostitutes, it adds.
The government has signed a European convention giving human trafficking victims more time to recover from their ordeal before deciding whether to help police.
The latest Home Office figures, from 2003, suggest at least 4,000 women from abroad have been forced into prostitution.
Mr Coaker said: "Frankly it brings tears to your eyes. It simply cannot carry on."
He said prostitutes' clients could face prosecution for rape: "If we have got a situation where a man knowingly has sex with a woman he knows is not freely consenting to that, then I think that that could be considered as rape."
The Home Office action plan calls for specialist trafficking teams to be set up at UK ports and airports.
It is also planning to set up an advice line for police, immigration staff and social workers and a national system to identify victims more quickly.
According to reports, "slave auctions" are being staged on the concourses of British airports.
Grahame Maxwell, of the UK Human Trafficking Centre, said information programmes were in place in countries like Bulgaria and Romania to prevent people being conned into going abroad.
One woman, imprisoned and forced to be a prostitute, had been raped 50 times on Christmas Day, the home office report said.
Another had been beaten to death after being held as a domestic slave and made to live in an outhouse, where she suffered from trench foot - caused by damp and wet conditions.
Being forced to have sex 30 times a day was common, Mr Maxwell said.
He added: "The average price for a trafficked woman is £2,000 to £3,000, but up to £8,000 is reported in some cases.
"We are talking about people here. These are prices that are being paid for individuals.
"There was £8,000 charged for two 15-year-old virgins from Lithuania."
Mr Coaker said trafficked children detected at Heathrow had started "kicking and screaming because they don't believe the police officer is trying to help them.
"They've been so deceived that if they don't get there will be big consequences."
Criminals had threatened to harm their families at home if they escaped, the government said.
The decision to sign up to the European agreement has cross-party backing.
Shadow home secretary David Davis said it was necessary for "moral reasons", such as protecting exploited victims, some of whom are forced to have sex with up to 40 men a day.
Liberal Democrat leader Sir Menzies Campbell said: "Slavery still exists in the UK and we must tackle modern-day slavery wherever we find it.
"The terrible reality is that women and children are falling victim to human trafficking."
There were an 4,000 victims of trafficking working in prostitution in the UK during 2003, according to Home Office estimates.
It is thought the problem has grown since then.