A crackdown on human trafficking has been launched by police forces across Britain and the Irish Republic.
The first Pentameter operation saw 84 trafficked women being rescued
It aims to rescue women and children forced to work in the sex industry and prosecute gangs who are profiting.
Home Secretary Jacqui Smith said she could not guarantee smuggled victims would not be removed but pledged to give them a 30-day reflection period.
The Poppy Project, which cares for such women, said it backed any police activity which gave victims a lifeline.
The operation, Codenamed Pentameter 2, involves all 55 police forces in the UK and the Republic of Ireland and the dedicated UK Human Trafficking Centre in Sheffield.
BBC correspondent Danny Shaw said three people had been rescued since the operation began on Monday with dozens more expected to follow.
The first Operation Pentameter in 2006 freed 84 women and teenage girls from brothels and massage parlours and led to 232 arrests.
It also led to the creation of the Human Trafficking Centre.
Gloucestershire Chief Constable Dr Tim Brain said the new initiative would aim to gather intelligence as well as enforce the law and rescue victims.
"It's to improve our picture but equally at the same time to ensure that we go after these gang masters, the people who are exploiting the women, and bring them to justice," he told BBC's Breakfast.
"It's a secret crime so nobody has a real idea of just how big it is. It's a very profitable crime for the criminals who work these women. We expect it to grow unless we do something about it."
The home secretary said recent research suggested 4,000 women in the country were working in areas of sexual exploitation and may have been trafficked.
She said there was no evidence to show the problem was getting worse but said the police operation would give a clearer picture.
2006 OPERATION PENTAMETER
84 women and teen girls freed
232 people arrested
515 premises visited
Women sold for £500-£8,000
Women and children were being trafficked from in and outside Europe, and within the UK too, she said.
"Two hundred years after we banned the trade in slavery in Parliament in the UK, it's shocking that this is still going on," she said.
At Wednesday's launch, Ms Smith said she shared concerns that British men who use prostitutes were driving the demand for criminal smuggling of women and girls.
But she added she did not support a change in the law to criminalise such men.
Border police needed
Shadow home secretary David Davis said human trafficking was a brutal trade which exploits the vulnerable.
"The key is prevention, which is why Conservative proposals for a dedicated border police force would have the greatest impact," he said.
The police have asking members of the public to play their part by reporting any suspicious behaviour to local police.
A new telephone information line has been set up to give advice to immigration staff and social workers attempting to identify victims of trafficking.
Denise Marshall, chief executive of the Poppy Project, said women and girls were being forced to have sex with up to 30 customers a day.
"We support any police activity that gives women who have been trafficked a lifeline."
Aidan McQuade, of Anti-Slavery International, has called on the government to commit to ensuring sufficient resources are made available to deal with the problem.
I manage an employment agency and I recruit large numbers of people from eastern Europe to work legitimately in this country. I've spent time in impoverished countries, I know how desperate their citizens are to come here and make a better life for themselves, and that can make them easy targets for traffickers. This is a fundamental issue for any society that considers itself civilised and just. I'm a disillusioned Labour voter, but if this government is ready to deal with people trafficking and commit the necessary resources over an extended period, then I'll vote for them no matter what else they do or fail to do.
David Jones, Swansea, UK
Well, rather than a crackdown, how about legalising prostitution? Other than the taxation benefits to the country, treat it as a business (it always will be there, like it or not) and police it properly. Any illegal business could face the same penalties as say a company not registering at Companies House, or a business not paying taxes etc. Just a thought.
Mike Daniell, Buckingham, Bucks
I just read an account from one woman in horror and noted that at the end the home secretary wouldn't give any assurances that trafficked women and children wouldn't be deported. This is absolutely disgusting, these women and children are victims of horrendous abuse. Often deportation could lead to them being tracked down in their native country by those who took them in the first place with them either being forced back into prostitution or even facing serious physical harm/death. As such they should be given the very same help and support a UK rape victim would be given and allowed to stay if they so wish.
ian candler, Chelmsford, Essex
Why is it a so called "secret crime"? Is it because it involves women and sex work? There clearly needs to be serious debate and discussion surrounding the topic of sex work so that it is no longer a "secret" crime as you call it - and the victims, the perpetrators and the public were given the freedom to discuss it openly.
Jessie Smith, Nottingham
I feel tremendous outrage that people-trafficking can happen in the UK and Europe in the 21st Century. Quite apart from that fact itself, I find it extremely shocking that so many men obviously use prostitutes and most shocking of all, that any man could use a woman for sex who is obviously drugged, distressed or in any other way unwilling. This has a name - it's called rape and there should be as many prosecutions of clients as there are for traffickers!
If people are really that concerned why don't the police target numerous escort agencies that operate throughout the country. Is it a legal loophole that prohibits them from doing this? I am sure most of the people involved in these things are living in this country illegally. Frankly speaking these agencies should be just forced to shut down and the illegal immigrants deported.
Arup Vidyerthy, London
How many hundreds of years has the sex trade been going? When will the various agencies realise it won't be stopped by "crackdowns"? Legalise it, control it, help make the girls safe by making it legal.
There is NO place for this crime, the people who exploit these women or the men who use them in this country! We should crack down on this ruthlessly. Give the women immunity from expulsion if they're not entitled to live here, put NO barriers in the way of them leaving their imprisonment and turning these evil criminals over to the police. Prosecutions should be done on the balance of probability here where the accused have been involved in serious or similar crimes. We cannot allow these people to get away with this terrible behaviour because both their victims are too scared to speak out and the surrounding secrecy. If we cannot imprison these thugs on this basis, take everything they have - make their crime a burden to them, DON'T let it pay.
Nathan Dale, Inverness, UK