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 flow of immigrants into Greece would lead the country, which is surviving on foreign loans, to collapse. “Our social fabric is at risk of unraveling,” Mr. 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 home on a chartered flight on Sunday, said the spokesman, who spoke on the standard ground rules of anonymity. He said more deportations were expected in the coming days.