US immigration officials in Southern California have detained hundreds of Iranians and other Muslim men who turned up to register under residence laws brought in as part of the anti-terror drive.
Reports say between 500 and 700 men were arrested in and around Los Angeles after they complied with an order to register by 16 December.
The Immigration and Naturalization Service (INS) is refusing to say how many people were arrested but said detainees were being held for suspected visa violations and other offences.
The arrests sparked angry protests in Los Angeles by thousands of Iranian-Americans waving banners which read "What's next? Concentration camps?" and "Free our fathers, brothers, husbands and sons".
Official radio in Iran also reported the arrests and the protests, which it said were mounted by families of the detainees who converged on Los Angeles.
Under the new US immigration rules, all male immigrants aged 16 and over from Iran, Iraq, Libya, Sudan and Syria had to register with authorities by Monday unless they had been naturalised as citizens.
Immigrants from other mainly Muslim states have been set later deadlines for registration.
Community groups said men had been arrested in Los Angeles and nearby Orange County as well as San Diego.
California is home to about 600,000 Iranians who have been living in exile since the 1979 Islamic revolution.
One of the Iranian-American demonstrators in Los Angeles, Ali Bozorgmehr, told the French news agency AFP that his community was being targeted unjustly.
"All Iranians that live in America are hard-working people ... They love this country and all ... are against terrorism," he said.
Ramona Ripston, executive director of the Southern California chapter of the American Civil Liberties Union, said the arrests were reminiscent of the internment of Japanese-Americans during World War II.
"I think it is shocking what is happening," she said.
"We are getting a lot of telephone calls from people. We are hearing that people went down wanting to co-operate and then they were detained."
Islamic community leaders said many detainees had been living, working and paying taxes in the US for up to a decade and had families there.
"Terrorists most likely wouldn't come to the INS to register," said Sabiha Khan of the Southern California chapter of the Council on American Islamic Relations.
She said the detainees were "being treated as criminals, and that really goes against American ideals of fairness, and justice and democracy".news.bbc.co.uk/2/hi/americas/2589317.stmE-mail this article