By Maayan Lubell

JERUSALEM (Reuters) - Israel's parliament has moved to ensure African migrants who enter the country illegally can be held without charge, despite a Supreme Court ruling that had struck down a previous detention law.

Legislation approved late on Monday set a maximum detention period of one year for new illegal migrants, a change from a term of up to three years stipulated in a previous law annulled by the court in September.

But with a newly-built Israeli border fence effectively choking off what had been a stream of African migrants crossing from Egypt, the new law could also have an impact on some of the estimated 50,000 mainly Sudanese and Eritrean nationals already in the Jewish state.

The new regulations, which opponents predicted would also be challenged in the Supreme Court, enables authorities to send migrants, now living illegally in Israeli cities, to what the government describes as "open facilities".

Under the law, their detention would be open-ended, pending resolution of their asylum requests, implementation of deportation orders or voluntary repatriation.

The first such complex, which can hold several hundred people, is due to begin operating this week in the southern Israeli desert.

Migrants detained there will be able to leave the facility during the day but must return at night, and they will not be allowed to seek employment. Women, children and families will not, at this stage, be sent to the complex, which the law stipulates must provide health care and social services.

Critics say the facility is effectively a prison.

Legislators who supported the new law said they were defending the Jewish character of Israel. Opponents called the measure undemocratic.

The government sees the migrants as illegal job-seekers, while rights groups and liberal lawmakers say many are asylum-seekers fleeing hardship and persecution in their homelands.

"This law is needed in order to deter potential infiltrators. The present reality is a human ticking timebomb," coalition lawmaker Miri Regev, head of the Knesset's Interior Committee, told parliament.

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Since the Supreme Court ruling in September, some 700 of the 1,700 migrants under detention have been released from a prison in southern Israel, officials said. The rest are to be transferred to the new "open facility" this week, the Prisons Authority said.

Tens of thousands of Africans, many working in low-paying jobs as cleaners and dish washers, still populate poor neighborhoods of Tel Aviv and other Israeli cities.

Israel has been trying to persuade them to leave voluntarily in return for a payout. Some 1,700 Sudanese and Eritreans have gone home this year, the Interior Ministry said.

Zahava Galon, head of the left-wing Meretz party, said the migrants were no threat to Israel's Jewish identity.

"Is this how we, as a people who have sought asylum, treat human beings?" she asked on Israel Radio.

(Writing by Maayan Lubell, Editing by Jeffrey Heller and Angus MacSwan)