Italy has threatened retaliation against the EU if governments don't make good on proposals to take in more asylum-seekers.
Hundreds of migrants desperate to reach northern Europe have refused for a third day to leave borders tightened by Italy's neighbours.
"We are not going back," chanted the migrants in Ventimiglia, on Italy's Mediterranean border with France. "Police, go away."
French border police blocked border crossings last week, citing the influx of migrants, and about 200 would-be refugees have refused to leave the rocks of Ventimiglia, just a few kilometres from the swanky resorts of Nice and Saint-Jean-Cap-Ferrat on the French Riviera.
Their protests came as officials ramped up diplomatic efforts to press the EU to show more solidarity with Italy, which has borne the brunt of rescuing migrants at sea and providing initial assistance to them at overflowing reception centres across the country.
Premier Matteo Renzi threatened an unspecified "Plan B" while his interior minister, Angelino Alfano, said the EU would find "a different Italy" if it doesn't endorse a refugee redistribution plan at an EU interior ministers' meeting this week in Luxembourg.
He said he would also press for European help repatriating migrants who don't qualify for asylum, saying Italy had borne the cost last year of sending back nearly 16,000 migrants passed over for refugee status.
"I will say with great clarity: Kids, either we do equal distribution of migrants in Europe, or we organise refugee camps in Libya, or we organise a serious policy of repatriation," Mr Alfano said. "If Europe doesn't follow through on its responsibility and solidarity, it will find a different Italy."
The European Commission had proposed EU governments endorse its scheme to share responsibility for taking in 40,000 recently-arrived Syrian and Eritrean would-be refugees. But diplomats in Brussels say nearly half of the EU nations are insisting that the scheme be voluntary, not mandatory.