JERUSALEM — Israeli authorities detained dozens of African migrants in predawn raids early Monday, the first major step toward what leaders say will be deportation of 4,500 people who have entered the country illegally.

The arrests were the harshest move yet against migrants, reflecting growing concern about the effect on Israel of tens of thousands of Africans who have sneaked into the country across the porous Egyptian border in recent years.

Israeli leaders have grown increasingly alarmed by the influx, calling it a burden and threat to the country’s Jewish character.

Interior Ministry spokeswoman Sabine Hadad said 55 people, including 45 from South Sudan, were rounded up in Monday’s raids, which took place in several towns across the country. More raids were planned in the coming days, she said.

In the Red Sea tourist town of Eilat, Channel 10 TV showed images of migrants quietly piling suitcases and belongings in the back of a police truck before being driven away.

The crackdown came in the wake of a landmark court decision last week that cleared the way for the expulsion of 4,500 migrants. The order applies only to people who come from nations that have friendly relations with Israel, most of whom are from the newly established state of South Sudan.

These people, however, represent only a small percentage of the estimated 60,000 Africans in Israel. Some claim asylum, while others are simply looking for work in a relatively prosperous country.

Because many are from Sudan, an enemy of Israel, and Eritrea, a country with a flawed human rights record, the line between refugees and economic migrants is blurred. Therefore, Israel has quietly allowed most migrants from those two countries to stay without processing their asylum applications.

The continued arrival of the Africans, and the legal uncertainty, have spurred action. Israel is racing to build a fence along its Egyptian border to halt the influx, and it has begun work on a huge detention center in its southern desert.

The government has begun imposing penalties on businesses that employ unauthorized immigrants, and this week, it announced that in addition to the detention center, it plans to build a tent city to hold 20,000 new arrivals.

Interior Minister Eli Yishai, who oversees immigration policy, told Army Radio that the first group of South Sudanese, about 100 people, will be sent home next week.