Simultaneous raids in four European countries in the early hours of this morning will "severely disrupt" the drug trade, officers hope.
As limits will soon be lifted on migrants from Romania and Bulgaria - a look at the economic argument for a rise in immigration.
The boss of Risby Park Farm said most of his workforce are Eastern European because British people aren't interested in working there.
A 22-year-old woman from Wales has died after a night out in a bar at a Bulgarian ski resort, officials have confirmed.
Nina Holmes collapsed outside BJ's bar in the town of Borovets, some 45 miles from the capital Sofoia, on Sunday night.
It has been reported that eight other British nationals were taken to hospital following a night out in the resort, prompting a police investigation.
A spokeswoman for the Foreign and Commonwealth Office confirmed the death of a British national in Bulgaria and said it was providing assistance to the family.
South Wales Police said it was aware of the incident and would be working with Swansea coroner Philip Rogers once Ms Holmes' body was repatriated.
The Bulgarian ambassador to the UK called the tone of the immigration debate "very unpleasant".
Konstantin Dimitrov told ITV News Political Correspondent Carl Dinnen, "It is defamatory, definitely there are elements of discrimination."
"This propaganda hurts the pride of many Bulgarians ... the humiliation that such a rhetoric contains is something that may leave a trace here or there in the soul of my compatriots," he added.
As flights and buses arriving today showed, many Romanians and Bulgarians have been living in Britain for years.
But we should also be welcoming new arrivals, just as Britons arriving in France, Spain and other European states find themselves welcomed.
– Natalie Bennett, leader of the Green Party of England and Wales
It's time to end the toxic immigration debate and acknowledge that, whether they are nurses and doctors coming to work in the NHS, computer game designers or building workers, Romanians and Bulgarians, as with other immigrants, will be contributing to our society.
Some will settle, some will only be here for a few years, and they reflect the mobile nature of life in the modern world which enriches all of our lives.
Victor Spiresau has arrived in the UK from Romania today in order to find a job in construction, he told ITV News.
The 30-year-old said he earned 10 euros a day working on building sites at home and that he hopes to make 10 euros an hour here, but was not planning on settling.
"I don't want to stay here. I want to renovate my home and to make a good life in Romania because it's much easier to live in Romania because it's not expensive."
Mr Spiresau said he already has work lined up washing cars in London but hopes to go on to work in the construction industry. He said he chose to come to the UK over other European countries as he can speak the language.
There are more British jobs advertised on a Romanian recruitment website than all of the other European countries with similar employment laws combined.
Eight other countries are joining the UK in lifting working restrictions for migrants today - Austria, Belgium, France, Germany, Luxembourg, Malta, Spain and the Netherlands.
The Romanian recruitment website Tjobs.ro carries the following numbers of job advertisements:
- UK - 4,896
- Austria - 245
- Belgium - 25
- France - 591
- Germany - 2,055
- Luxembourg - 50
- Malta - 95
- Spain - 130
- Netherlands - 214
Tory MP Mark Reckless accompanied Keith Vaz at Luton Airport, claiming he was concerned that large numbers of Romanians and Bulgarians may come to the UK.
– mark reckless, tory mp
I'm here this morning because my party - the Conservatives - we made a promise that we would cut immigration from hundreds of thousands to tens of thousands a year.
And my concern is that if we do see large numbers coming from Romania and Bulgaria then that could knock us off that target and the promise we made to the electorate.
Romanians landing at Luton Airport were greeted by Home Affairs Select Committee chairman Keith Vaz on the first day of new rules allowing the migrants to work in the UK.
He said he saw no evidence of migrants who have "rushed out and bought tickets" because of the lifting of regulations.
He said that most of the people he met already work in the UK and were "coming back after a holiday".
But he also criticised the government for a "lack of robust estimates" of the number of Romanians and Bulgarians expected to arrive in the UK.
British companies are turning to Romanian workers to fill job vacancies as restrictions on the UK labour market are lifted from today.
Employers are advertising nearly 5,000 positions on Tjobs.ro, a Romanian recruitment website which claims to help 160,000 find work.
Some 4,896 jobs were advertised today as being available in England, along with 80 in Northern Ireland, 24 in Scotland and 20 in Wales.
Among them are 50 nursing positions in southern England, 100 private hire taxi drivers, 10 GPs in Liverpool and 20 carpenters in London.
The Government has done "everything possible" to ensure people come to the UK for the right reasons, the Home Office has said.
Speaking ahead of today's rule change, a Home Office spokesman said, "Hard-working people expect and deserve an immigration system that is fair to British citizens and legitimate migrants and tough on those who abuse the system and flout the law.
"We welcome those that want to come here to work and contribute to the economy, but no EU national has unrestricted access to the UK - they must be working, studying or self-sufficient.
"Across Government, we are working to ensure that our controls on accessing benefits and services, including the NHS and social housing, are amongst the tightest in Europe to protect the UK from abuse."