"Unfortunately we will have to keep the Italian Roma because we can't expel them", Salvini, who is head of the far-right League, told television station Telelombardia.
The Aquarius itself entered the port shortly after 09:30 carrying another 106 migrants.
Most Spaniards support the idea of welcoming and helping to integrate refugees, pollsters say, allowing Sanchez, a Socialist, to offer migrant-friendly policies to voters who feel previous governments did not do enough.
Immigration will be a top issue at the European Union leaders' June 28-29 summit.
The OpinionWay poll was carried out among 1,020 adults online from June 13-15.
In Spain the migrants were granted authorisation to remain in the country for 45 days while each individual's legal case is studied. Some of the migrants were transferred from the Aquarius to the two Italian boats.
An aid group's ship and two Italian military vessels docked Sunday at the Spanish port of Valencia, ending a weeklong ordeal for hundreds of people who were rescued from the Mediterranean Sea only to become pawns in a European political fight.More news: Yemen pro-govt forces seize Hodeida airport from rebels: coalition
Dattilo, with around 270 migrants on board, was the first of the three boats to arrive at Valencia before 7 a.m. local time (0500 GMT) on Sunday.
As it neared port, those on board had danced and sang, their excitement captured in footage released on Twitter by SOS Mediterranee which operates the vessel together with Doctors Without Borders (MSF). It took the convoy almost a week to travel the 1,500 kilometers (930 miles) between Sicily and Spain.
Doctors Without Borders said the migrants come from 26 countries, mostly from sub-Saharan Africa, but also from Afghanistan, Bangladesh and Pakistan.
Italy's hard-line interior minister, who recently sparked a multinational showdown by refusing entry to a Mediterranean Sea rescue boat packed with 630 migrants, is now taking aim at Italy's minority Roma community.
"They will be dealt with according to our laws, without exception", Spain's minister of public works, Jose Luis Abalos, said Saturday.
The refusal by Italy and Malta to allow the Aquarius to dock has reignited a continentwide battle over how to handle immigration.
The Aquarius originally picked up about 630 migrants - including pregnant women and children - from six rubber dinghies in the Mediterranean Sea off Libya.
Spain has accepted the French government's offer to take in those migrants who want to go to France "once they have fulfilled the protocols". Through the first five months of 2018, some 35,455 migrants reached European shores, with 11,792 of them arriving in Spain.