The head of the American Muslim Foundation, Abd al-Rahman al-Amoudi, was arrested on Monday, Aljazeera’s correspondent in Washington reports.
Muslim and human rights groups have been waiting for more information to emerge before commenting in detail.
But the head of the International Solidarity Organisation for Human Rights, Yasir Bu Shnaq, told Aljazeera that al-Amoudi enjoyed a good reputation among Muslims both in the US and abroad.
He said al-Amoudi was widely known as "a moderate", and doubted the AMF head had broken any American or other laws.
Al-Amoudi worked with the US State Department under the Clinton administration in the 1990s as a goodwill ambassador to Muslim countries and was often in touch with the White House.
However, he has frequently been condemned by right-wing and pro-Zionist critics for expressing support for the aims of armed groups such as Hamas and Hizb Allah, becoming a hate figure for the US Zionist lobby in particular.
Al-Amoudi’s case is the latest example of a post 9/11 crackdown against members of Muslim organisations in the US.
His arrest follows repeated calls by pro-Israeli politicians, notably Democrat Senator Charles Schumer, for US authorities to be more vigilant towards Muslims with suspected links to political groups in the Middle East.
Earlier this month, the Wall Street Journal reported a senior figure in the American Muslim Council - which al-Amoudi used to head - had been held since June on suspicion of helping to finance groups regarded as international terrorists.
The business daily newspaper said an Egyptian AMC advisory board member, Sulayman Biheiri, had been detained for suspected links to the opposition Muslim Brotherhood movement in Egypt.
"The case of Soliman Biheiri has become the spearhead of a two-year investigation into whether Islamic [sic] activists and their wealthy Saudi backers assembled an empire of dozens of well-funded businesses and charities in this country to support terrorists and their causes," the paper said.