Israeli authorities have detained dozens of South Sudanese migrants ahead of their deportation, as part of a clampdown on illegal migrants.

Authorities started rounding up the African migrants on 10 June, three days after a court ruled their lives were not at risk in their homeland.

"The deportation operation is getting under way. We are starting the job," interior minister Eli Yishai told independent television station Channel Two.

"We told the infiltrators from South Sudan to come voluntarily; whoever doesn't, with the Lord's help, we shall get them all," he said, adding, "They'll be put on a plane."

Authorities are said to have arrested between 22 and 55 migrants, most of them Sudanese. Other migrants being detained are from Nigeria, Ghana, Ivory Coast, Romania and Sri Lanka.

Activists criticised the move, saying most of the arrests were made during raids.

Hundreds of African migrants protested in south Tel Aviv, with some even calling on the United Nations to intervene on their behalf.

Others held up banners that read: "An African is a human being."

Interior ministry figures show that approximately 60,000 African immigrants have entered Israel illegally, of which about 1,500 of them are from South Sudan.

Israel announced on 3 June that migrants who crossed the border illegally could now be detained for up to three years.

Human Right Watch has urged Israel to amend the law.

"Subjecting irregular border-crossers to potential indefinite detention without charge or access to legal representation would violate the prohibition against arbitrary detention under international human rights law," the New York-based group said.

The rights watchdog said the law could further bolster sentiment against immigration, just weeks after an anti-immigrant protest in Tel Aviv turned violent.

African migrants have been targeted by a series of attacks in recent weeks.

"Israeli officials are not only adding rhetorical fuel to the xenophobic fire, but they now have a new law that punishes refugees in violation of international law," Bill Frelick, director of the organisation's refugee programme, said in a statement.

"The law should be amended immediately and not enforced until necessary revisions are made."

In February, Israel's interior ministry announced plans to build an immigrant detention centre in the south of the country.