In the most significant challenge yet to the centre-left Italian government's policy of distributing across the country the thousands of migrants arriving at southern ports, Virginia Raggi said it was "time to listen to the citizens of Rome."
"We cannot permit the creation of more social tensions," Raggi, a member of the populist Five Star Movement, wrote on her Facebook page.
"That is why I am saying it is impossible, risky even, to think about creating any new reception structures."
Her appeal is likely to fall on deaf ears. The ministry is asking all Italian municipalities to expand their reception facilities for the expected arrival of around 250,000 migrants over the course of 2017, compared with 181,436 last year.
With existing facilities jammed full, Italy is dealing with a mounting backlog of recently-arrived migrants as a result of neighbouring states tightening their borders, making it harder for new arrivals to reach preferred destinations in northern Europe.
As of Monday, Italy had registered 61,903 new migrants in 2017, up from 52,275 in the same period of 2016. Arrivals generally peak over the summer months when sea conditions make it easier for traffickers to launch boats from Libya.
More than half a million people from Africa, South Asia and the Middle East landed in Italy between the start of 2014 and the end of 2016.
Processing records suggest at least 40 percent of them have a valid claim to asylum in Europe or leave to remain on humanitarian grounds.
The others, classified as illegal economic migrants, are liable to be deported but that is often difficult to organise because their countries of origin frequently refuse to take them back.
Grillo vows Roma crackdown
It is not the first time Italy's government has been confronted by a local authority threatening not to take its share of migrants.
Previous challenges have been faced down by threats to cut funding from central government. But Rome's move could inspire other major urban centres to follow suit, particularly in northern Italy, where the anti-immigration Northern League is very influential.
Raggi's move came on the same day that Five Star's leader, Beppe Grillo, announced a crackdown on ethnic Roma migrants from eastern Europe living in makeshift camps in the Rome area.
"Now the music in Rome changes," Grillo wrote on his blog. "Anyone who declares themselves penniless and drives a luxury car will be out. Anyone who begs in the metro with kids in tow, is out. Surveillance against pickpockets in the metro will be increased."
Political rivals said Grillo's outburst and Raggi's move represented a tilt to the right in the populists' agenda after the young party's advance stalled in local elections at the weekend. The polls setback cast doubt on its ability to win national power in the next general election, expected in early 2018.
The pressure on Italy's refugee system has been exacerbated by its EU partners reneging on promises to take in asylum seekers under a scheme aimed at relocating 160,000 people from Italy and Greece to other EU states.
As of June 5, only 5,694 people had been relocated from Italy and several eastern European countries are refusing point blank to comply with the scheme.
The European Commission on Tuesday began legal action against three of them, the Czech Republic, Hungary and Poland, over the issue.
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