Citing growing concern that the millennium will bring thousands of unstable visitors to the Holy Land, the Israeli government today rounded up 21 foreign-born members of two Christian groups and considered whether to expel them.

The arrests and the likely deportations are part of a crackdown on groups seen as posing a risk of violence at the new year, according to government officials. A police spokeswoman said the Christians, including 16 Americans, were possibly planning actions that could threaten public safety, although she gave no details.

"I wouldn't say any of these people were actually dangerous. Probably they were not. But these days if there is even the potential for something, you don't wait," said a senior official who spoke on condition of anonymity.

The arrests marked the third time this year that the government has moved against Christians suspected of being extremists or members of a cult. Israeli officials have said that as 2000 approaches, extremist Christians may act violently to realize their interpretation of biblical prophecy.

The Christians, eight of them children, were arrested on grounds that their visas had expired. Indeed, an American known as Brother David, who led one of the groups, has said that as a Christian he is a citizen of the world and sees no need to have even a passport.

Still, American Christians who know the people who were arrested said they fear the government may crack down on foreigners who simply seem unconventional.

"Are the police going to arrest Christians? That's what I'm worried about," said David Bogenrief, who said he is a Christian musician who has lived in Israel for 15 years. "We sing and evangelize and we do it in a way that some people may think is crazy."

In a recent interview, Brother David said he came to Israel 20 years ago after being born again when God spoke to him while he was cleaning a Methodist church in Syracuse, N.Y. With that, he said, he flew to Israel and hitchhiked to Jerusalem.

About five years ago, he said God told him to move to the Mount of Olives and prepare for the coming of the Messiah. He said that by 2000 he hopes to attract as many as 500 born-again Christians to be with him and have what he called "front-row seats" for the return of Jesus.

Brother David and another American arrested today, called Sister Sharon, rented inexpensive apartments to their followers and others on the Mount of Olives, a place populated by Palestinians. Some of their neighbors described them as eccentric but not dangerous.

"I never saw anyone with a problem from them," Farida Abu Doula said. "They gave the children here used clothes they got from the United States. They gave some to my children too."