ATHENS — A vast police operation here aimed at identifying illegal immigrants found that, of 6,000 people detained over the weekend, 1,400 did not have proper documentation, leading the minister of public order to say that Greece was suffering an ''unprecedented invasion'' that was threatening the stability of the debt-racked nation.
The minister, Nikos Dendias, defended the mass detentions, saying that a failure to curb a relentless influx of immigrants into Greece would lead the country, which is surviving on foreign loans, to collapse.
''Our social fabric is at risk of unraveling,'' Dendias told a private television channel, Skai. ''The immigration problem is perhaps even greater than the financial one.''
He said he would resign if he was obstructed. ''There would be no point in me staying on,'' he said. That appeared to be a warning to left-wing opposition parties, one of which called the operation a pogrom.
About 4,500 officers conducted raids on streets and in run-down apartment blocks in central Athens, a police spokesman said, calling the sweep one of the largest ever by the force.
Eighty-eight Pakistanis were flown back to their homeland on a chartered flight on Sunday, said the spokesman, who spoke on usual ground rules of anonymity. He said many more deportations were expected in the coming days.
With its position on the southeastern flank of the European Union, Greece has long been the most common transit country for impoverished migrants from Africa, Asia, and the Middle East. But the global economic malaise and the revolutions of the Arab Spring have sharply increased the flow of migrants, and the government has been calling for more help from the EU.
Prime Minister Antonis Samaras promised to crack down on illegal immigration in campaigning before the general elections in June that his conservative New Democracy Party won by a small margin, followed by Syriza, the party that denounced the weekend operation. But no mass efforts had been made before that, fueling the fury of ascendant right-wing groups.
Last week, the authorities decided to transfer hundreds of officers to Greece's land border with Turkey, a popular route for smugglers sneaking migrants from Africa and Asia into the country for a fee.
Many of those officers have been moved to border guard duty from the security details of politicians as part of an overhaul of the force.
The reinforcements were sent amid fears of an increased influx of refugees from Syria, where political tumult has devolved into civil war.
The growing population of immigrants in Greece — about 800,000 are registered, and an estimated 350,000 or more are in the country illegally — adds to the anxieties of many Greeks, who are seeing the government's once-generous social spending evaporate. They complain that the foreign residents are depriving them of jobs and threatening the national identity.
Such frustrations have been exploited politically, notably by Golden Dawn, a far-right group that has been widely linked to a rising number of apparently racially motivated assaults but vehemently denies being a neo-Nazi group.