The US Muslim and Arab communities, still grieving over the events of terror last week, are dealing with hundreds of assaults, bomb threats and harassment.

Many Muslim women are frightened to leave their homes with their heads covered and children are scared to go to school. But the communities have been heartened by what they see as unstinting support from the Bush administration, Congress and the media. They were particularly encouraged by President George W. Bush's visit on Monday to the Islamic Center of Washington, where he denounced those seeking reprisals as "representatives of the worst of humankind".

The police have been swift to launch investigations into attacks on Muslims in the past week. And the Department of Justice is preparing prosecutions for hate crimes within the next two weeks, according to James Zogby, chairman of the Arab American Institute in Washington.

But the authorities are under unprecedented pressure to clamp down on terrorist activity, and Arab and Muslim Americans fear that there are already signs of over-zealousness and erosion of their civil liberties.

Stanley Cohen, a Jewish civil rights lawyer, who defends many Muslims, said the FBI had intimidated worshippers at mosques and entered their homes "at all hours of the day and night".

"When they are less than co-operative, they have called in [the immigration service] to investigate them," he said. "We have seen an unprecedented increase in profiling." He noted that the FBI and local police have been picking out cars driven by Arabs, stopping them as they come in and out of the major thoroughfares of Washington and questioning the drivers.

He claims that illegal seizures of property by the police have been proliferating and a large number of "material witnesses" have been jailed.

John Ashcroft, the attorney general, has vowed to protect the civil rights of "everyone". Administration officials from the state and justice departments have been regularly meeting Arab and Muslim spokesmen to hear their concerns.

The Civil Rights Commission and the Equal Employment Opportunity Commission have dispatched directives to their local offices to make special efforts against discrimination against Arabs and Muslims.

However, the administration is committed to "smoking out" the terrorists. The Justice Department has announced rules governing detention that would allow immigrants to be held indefinitely during a national emergency, instead of the current 48-hour duration. A broad package of anti-terrorism legislation will be sent to Congress to give the Justice Department powers to arrest immigrants.

"It's starting," said Ibrahim Hooper of the Council on American Islamic Relations. "The environment of hysteria will lead to the curtailment of civil liberties."

His group has received notice of more than 400 instances of harassment and assaults against Muslims. Incidents of over-reaching police tactics could hinder the efforts by the administration to include Muslims and Arab governments in an anti-terrorism coalition. The FBI raided and "tore up" the apartment of the son of a prominent Saudi, Mr Zogby said, before returning two more times for questioning.

Local police agencies have also allegedly been "over-zealous". Driving home late at night recently, Mr Zogby recognised an elderly Pakistani with a white beard backed against a wall by five or six police cars.

The FBI declined to comment last night on claims about over-zealousness by the law enforcement agencies.

"Because of the nature of the challenge and the response that is planned [against terrorism]," the US could not afford to allow such incidents, said Clovis Maksoud, former Arab League ambassador to the UN and the US. It was important not to reinforce "elements in the Muslim world" already opposed to US policies.

The administration's sensitivity to Muslim and Arab concerns comes after years of organising. Many groups lobby in Washington. The Council on American Islamic Relations was founded in 1992 by two people to counter stereotyping of Muslims by the American media and pop culture. It now has branches around the country engaged in grassroots activism against companies or media that portray them offensively.

Most Muslim commentators have praised the sensitivity of US television anchors - particularly Ted Koppel, who frequently urged viewers not to confuse Muslims and Arabs with the terrorists.

But, said Mr Molavi, some pundits have used incendiary language. Bill O'Reilly, a talk show host on cable TV, urged the bombing of Afghanistan, Libya and Iraq, saying: "If they don't like it, they can eat sand".

Most Muslim and Arab American leaders say they want what they seem least likely to get: a serious discussion of the roots of terrorism.