Police have smashed an international people smuggling ring in an operation which also saw raids by police in UK, Italy, France, Greece and Turkey.
Police said most of the immigrants came from Iraq and Afghanistan
Authorities said the migrants came from the Middle East, with most ending up in Britain and some in France.
Police said dozens of people were arrested in the five countries, following a three-year investigation.
They said the gang made millions of euros trafficking more than 5,000 people to Italy via Greece.
Police said they were mostly trafficked in boats or lorries.
The immigrants are said to have paid up to $15,000 (£8,500) for their passage, with around half of that for the France to Britain leg.
Police in Italy said the network was set up five years ago and run by an Iraqi Kurd, 30-year-old Ali Ako - nicknamed Arsalan - who made several million dollars.
"He managed the global strategy of the entire organisation from the [Italian] capital," they said.
"The immigrants and their families... were forced to pay huge sums to get to Europe, or, in rare cases, North America."
Paris prosecutor Jean-Claude Marin told a news conference in the French capital that 53 people were arrested after the three-year investigation.
He said 22 of them were in France, 18 in Italy, seven in Britain, three in Turkey and three in Greece.
It was "the biggest-ever simultaneous operation ever conducted on an international level" by agencies tackling people-smuggling, Mr Marin said.
Police in Italy said Iraqi Kurd, Ali Ako, ran the network there.
"It is one of the most important, if not the most important, operation dismantling clandestine immigration networks in France."
"The organisers ended up having a monopoly on the passage of the clandestine immigrants, of all nationalities, to Great Britain," said Denis Pajaud, of France's office fighting illegal immigration, OCRIEST.
French police said one of those arrested in the UK was the alleged mastermind of the ring.
Mr Marin said the man would be extradited to France by early January.
UK Immigration Minister Tony McNulty said it was another "excellent example" of cooperation between the UK and EU authorities.
He said: "We are committed to taking robust action against those involved in the facilitation of illegal entry and the distress it causes.
"There will always be those determined to try to enter the UK, hence the importance of police operations to target the criminal networks seeking to profit from this."
Thousands of illegal migrants try to enter Italy every year, often aboard rickety boats.
Last month Italian authorities found the bodies of nine migrants washed up on the south-east Sicilian coast after their boat ran aground off the shore.