Protests have taken place across Spain calling for the release of Wikileaks founder Julian Assange, who is facing extradition from the UK to Sweden for alleged sexual offences.
Hundreds of people gathered outside the British embassy in Madrid calling for him to be freed.
Wikileaks is publishing insights from hundreds of thousands of sensitive US diplomatic and military documents.
The demonstrators believe Mr Assange's detention is politically motivated.
The whistle-blowing website has angered and embarrassed governments around the world through its publication in recent weeks of classified US diplomatic cables.
Mr Assange was detained in London on Tuesday after Sweden secured an international warrant for his arrest.
Prosecutors in Sweden say they want to question him in connection with the sexual offence allegations. He was refused bail by a British court and has said he will fight extradition.
There have also been calls from some in the US for his arrest and prosecution on charges related directly to Wikileaks' activity.
While supporters online have mounted cyber-protests against Mr Assange's detention, Saturday's protests were some of the first street demonstrations in support of Wikileaks.
Wearing face masks associated with the "Anonymous" group of hackers - which launched cyber attacks after Mr Assange's arrest in the UK - the crowd in Madrid shouted for his freedom, outside the vast glass tower that houses the British embassy
Many of the demonstrators were angry at some of the revelations in the cables, says the BBC's Sarah Rainsford in Madrid.
These include the suggestion Spain came under pressure to stop a criminal investigation into the killing of Jose Couso, a Spanish cameraman who died when American soldiers fired a tank round into his hotel in Baghdad.
The Free Wikileaks website, which organised the demonstrations, said protests were also planned for other Spanish cities, including Barcelona, Valencia and Seville.
It called for the full restoration of Wikileaks' website, which was denied hosting services by Amazon after the first of the leaked cables were published two weeks ago.
The protest organisers also demanded that Visa and MasterCard restore credit card services because, it said, no one had proven Mr Assange's guilt.
Our correspondent says the issue of freedom of speech is sensitive for Spaniards, who only emerged from four decades of authoritarian rule in the 1970s.