Greece has long been Europe's main entry point for illegal immigrants from Asia and Africa seeking a better life in the West. But Greece's severe economic problems and high unemployment are making the problem worse than ever.

Police said Monday that 6,000 people were detained over the weekend in Athens in a massive operation incongruously named after the ancient Greek god of hospitality, Zeus Xenios.

Officers across the city were seen stopping mostly African and Asian people in the street for identification checks. Most were only briefly detained, but about 1,600 were arrested for illegally entering Greece and sent to holding centers pending deportation.

Left-wing opposition parties criticized the crackdown, while the Office of the United Nations High Commissioner for Refugees voiced concern that migrants from war-torn countries and genuine asylum-seekers could be denied the right of protection.

Some 100,000 illegal immigrants are estimated to slip into Greece every year, mostly from neighboring Turkey, and up to a million are believed to live in Greece, which has an official population of about 10 million.

The uncontrolled influx, which coincided with a recent spike in crime, contributed to the sharp rise of an extreme-right political party which uses aggressive rhetoric against immigrants.

Once beyond the pale of Greek politics, the extreme right Golden Dawn gained nearly 7% of the vote in parliamentary elections six weeks ago. Mainstream parties also pledged to curtail immigrant flows.

Public Order Minister Nikos Dendias said Monday the rounding-up of illegal immigrants would continue, arguing that their unchecked entry has brought Greece "to the brink of collapse."

"The country is being lost," he told private Skai TV. "What is happening now is (Greece's) greatest invasion ever. Since the Dorian invasion some 3,000 years ago, the country has never received such a flow of immigration."

Ancient tradition linked the invasion of Greek-speaking Dorian tribes with the end of the heroic Mycenaean age, although historians believe that the Mycenaean palatial civilization was brought down by financial and social unrest.

Dendias said arrested immigrants will be temporarily held at police academy buildings in northern Greece, which are closed for the summer, and at a detention center outside Athens. He claimed that by the end of the year Greece will be able to detain up to 10,000 people.

"Whoever is arrested will be held and then deported," he said.

The Greek office of the U.N. High Commission for refugees said that while Greece has the right to carry out checks on immigrants, it should ensure that vulnerable groups do not suffer. "People who truly need protection must be able to request it," said Petros Mastakas, associate protection officer at the UNHCR office in Athens.

"It is very difficult, practically impossible, for asylum seekers to apply for protected status, and we are concerned that among those arrested there may be people who want protection but were unable to submit their requests because access to the relevant authorities is practically impossible," he said.