Protests will be held around the world today against the detention of Julian Assange, the founder of WikiLeaks.

Demonstrations are planned in the capitals of Spain, the Netherlands, Colombia, Argentina, Mexico and Peru to demand Assange's release, the re-establishment of the WikiLeaks domain name and the restoration of Visa and Mastercard credit services to allow supporters to donate money to the whistleblowing site.

A statement on the Spanish-language website Free WikiLeaks said: "We seek the liberation of Julian Assange in United Kingdom territory." The website called on protesters to gather at 6pm (17.00 GMT) in Madrid, Barcelona, Valencia and Seville and three other Spanish cities.

It also calls for "the re-establishment of the WikiLeaks (wikileaks.org) internet domain," and the restoration of Visa and MasterCard credit card services to enable the "freedom to move money" because no one has "proved Assange's guilt", nor charged WikiLeaks with any crime.

Assange is in Wandsworth prison in south London after being refused bail on Tuesday. Sweden is seeking his extradition over allegations of sexual assault.

His lawyers said yesterday they were preparing for a possible indictment by the US authorities.

Jennifer Robinson said her team had heard from "several different US lawyers rumours that an indictment was on its way or had happened already, but we don't know".

According to some reports, Washington is seeking to prosecute Assange under the 1917 act, which was used unsuccessfully to try to gag the New York Times when it published the Pentagon Papers in the 1970s. However, despite escalating rhetoric over the past fortnight, no charges have yet been lodged, and government sources say they are unaware any such move is being prepared.

Robinson said Assange's team did not believe the US had grounds to prosecute him but understood that Washington was "looking closely at other charges, such as computer charges, so we have one eye on it".

Earlier this week, the US attorney general, Eric Holder, said the US had been put at risk by the flood of confidential diplomatic documents released by WikiLeaks and he authorised a criminal investigation.