Thirteen Moroccan migrants held in a reception centre in Rome for more than two months have sewed their mouths shut in protest at the length of their detention, police and campaigners said today.

Italy has struggled to absorb thousands of immigrants in recent years, many of them fleeing the civil war in Syria and travelling through North Africa to the Mediterranean island of Lampedusa on flimsy boats. More than 40,000 arrived last year.

The issue has become a political battleground. Rights groups have complained about the conditions in centres while the anti-immigration Northern League has attacked the government, saying it was being too generous to new arrivals, especially at a time of recession.

The thirteen, who arrived in October and were taken to the Ponte Galeria centre the following month, sewed their mouths shut on Saturday night, repeating a protest they staged at the end of last year, said police.

"They've been left in complete uncertainty, no-one has explained anything to them," said Gabriella Guido, spokeswoman for LasciateCIEntrare, a protest group set up to highlight the tough conditions in the migrant reception centres.

Guido, who spoke to the men late on Saturday, said there was a tense climate in the centres, which were only designed for short stays. "They are under stress, angry and they've lost faith. This is the only form of protest they have."

"It's one thing if you have to put up with it for a month or so, it's very different when it's eight, nine months without any sign of when it's going to end," she added.

Their earlier demonstration coincided with a video released online showing migrants standing naked in the cold while they were sprayed for scabies.

Campaigners say most migrants want to go to other European countries but can find themselves stuck in limbo in Italy.

On Sunday, Northern League Senator Massimo Bitonci said the migrants were being treated over generously in comparison with Italians struggling in the recession.

"In a country where there are no resources you wonder what services the interior minister has had to cut to provide this benefit to the migrants," he said in a statement.

His groups has opposed moves by the coalition government of Prime Minister Enrico Letta to lift criminal penalties on people caught entering the country without authorization.