Hundreds of protesters rallied outside FBI offices in Minneapolis and Chicago on Monday to condemn recent raids on homes and offices of anti-war activists in both cities.
On Friday FBI agents ransacked five homes of anti-war activists in Minneapolis, along with the offices of the Minnesota Anti-War Committee.
They also searched two activists' homes in Chicago, where agents confiscated computers, mobile phones, papers and financial records.
It is believed that the authorities were seeking to uncover links between the activists and groups including the Revolutionary Armed Forces of Colombia (Farc), the Popular Front for the Liberation of Palestine (PFLP) and Hezbollah - all groups dubbed "terrorist" by Washington.
Peace campaigners ridiculed the suggestion that they might have provided material support to terrorism and denied contributing money to terrorists.
One of the homes searched was that of Jess Sundin of the Minnesota Anti-War Committee.
She told protesters that she knew of 13 people subpoenaed to appear before a federal grand jury in Chicago next month.
She and two other Minnesotans who were searched - Mick Kelly and Meredith Aby - acknowledged that they had met like-minded activists while travelling in the Middle East or Colombia. But they all denied contributing any money to terrorist groups.
Ms Sundin and Mr Kelly are both members of the Freedom Road Socialist Organisation, a group that "openly supports Farc and the PFLP and shares their Marxist ideologies," according to the FBI.
But Mr Kelly denied offering the group material support. "I can't stress that long enough or loud enough, and honestly I don't believe that's why we're facing this scrutiny, " he said.
The activists searched in Minnesota were involved in organising the mass anti-war march at the the 2008 Republican national convention in St Paul.
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