The mayors of three large Italian cities are refusing to obey a controversial anti-immigration law penned by far-right deputy prime minister Matteo Salvini, slamming it as unconstitutional.

Salvini, also interior minister, on Thursday, demanded the resignations of the rebellious leaders of Florence, Palermo and Naples, with the last escalating the row by also offering to take in migrants stranded at sea that Italy has turned away.

"This (law) incites criminality, rather than fighting or preventing it," Palermo mayor Leoluca Orlando said.

"It violates human rights. There are thousands, tens of thousands of people who legally reside here in Italy, who pay their taxes, who pay into pensions, and in a couple of weeks or months they will become... illegal," he claimed.

The tough new anti-migrant law, adopted by parliament on November 28, makes it easier to expel new arrivals and limits residence permits in a country that has become the main gateway for migrants crossing the Mediterranean.

It also abolishes humanitarian residence permits granted until now to vulnerable people, families or single women with children.

Salvini, head of the far-right League party, was quick to suggest Orlando "take care of the many problems in his city instead".

But his rebuke proved counterproductive with other mayors joining in the protest.

- 'We will not bow' -
Florence mayor Dario Nardella said his city would "not bow to" a law which "expels asylum-seekers and, without repatriating them, throws them out onto the street".

Naples mayor Luigi de Magistris vowed that the parts of the law that he said were unconstitutional, "such as those on the right to asylum, will under no circumstances be enforced".

He then upped the ante by offering to take in 32 migrants who are blocked in limbo at sea after being rescued by an NGO but denied a safe port in Europe.

- 'Heart of stone' -
"I hope the boat approaches the port of Naples, because -- contrary to what the government says -- we will launch a rescue plan and let them dock. I will oversee the rescue operation myself," he told a local radio.

He accused Salvini of having "a stone instead of a heart".

Salvini snapped back on Facebook that those who helped illegal migrants "hate Italians and will answer to the Law and History".

Orlando said he would seek legal advice on whether the law was unconstitutional. 

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