The raids were carried out by 400 officers in Britain, France, Greece, Italy and Turkey on Wednesday working under Interpol and Europol coordination.
French prosecutor Jean-Claude Marin said it was "the biggest-ever simultaneous operation ever conducted on an international level" by law enforcement agencies cracking down on people-smuggling.
"The organisers ended up having a monopoly on the passage of the clandestine immigrants, of all nationalities, to Great Britain," said Denis Pajaud, the head of France's office fighting illegal immigration, OCRIEST.
French authorities said most of the illegal immigrants came from Afghanistan and the Kurdish part of northern Iraq. Some also came from Pakistan, Somalia and Ethiopia.
Arrests
The people-smuggling ring had been operating for the past five years and had succeeded in getting around 4,000 to 5,000 clandestines into Britain and other northern European countries, Italian and French officials said.
French authorities said a total of 53 people were arrested, 22 in France, 18 in Italy, seven in Britain, three in Turkey and three in Greece.
They said one of those arrested in Britain was the alleged mastermind of the ring, who had a European arrest warrant issued by France against him.
Italian police put the number apprehended in the five countries at 90, including an Iraqi Kurd in Rome who they said had pocketed millions of dollars from the illicit business.
The reason for the discrepancy between the totals given by the two countries' authorities was not immediately known.
"Among the 90 people arrested, 35 were for links with the Mafia," Piero Grasso, Italy's chief Mafia prosecutor said.
Gilberto Caldarazzi, an Italian police spokesman, told a news conference that the swoops were the result of three years of investigation.
The immigrants went from Turkey to either Belgium, Greece, Italy or Switzerland before arriving in Paris.
They rested in parks near the city's Gare du Nord railway station, and then headed to France's Calais region near the English Channel for the passage into Britain.
Some jumped inside trucks without the drivers knowing.
A minority of them ended up staying in France, or going to Germany or Scandinavian countries.
In those cases, they gathered in the French town of Oyannax, near the Swiss border.
Inhuman conditions
Mr Grasso said the immigrants were transported "in inhuman conditions in containers, or lorries, and were treated like goods."
French officials said the immigrants paid 6,000 to 7,000 euros (US$7,000 to US$8,200) for their passage.
Italian authorities put the fee at between US$7,500 dollars and US$15,000 dollars.
According to French police, the network was very structured. In France, they said, it was run by a chief who had underlings who in turn managed teams of touts, escorts, smugglers and people providing lodgings.
All of them were Iraqis, Iranians or Afghans with valid papers to live in France.
Mr Marin said the alleged mastermind and two of the others arrested in Britain believed to be his lieutenants, who also had European warrants from France against them, would be extradited to France by early next month.
British police found bank documents and equipment to make fake Iraqi and Iranian identification documents at the alleged ringleader's British address, French officials said.
Two investigating judges, Jean-Louis Peries and Baudoin Thouvenot, are heading a preliminary criminal probe in France against the ring.