Lawyers in Britain seeking to block access to classified material seized from a Brazilian held at Heathrow airport have argued his detention is unlawful. The judicial proceedings against David Miranda -- partner of The Guardian journalist Glenn Greenwald who reported on the massive U.S. covert monitoring system -- involved the police and Home Office, which told the court \"tens of thousands of highly classified documents\" were seized, the BBC reported Thursday. Miranda, 28, was held for nine hours at the airport Sunday under a provision of Britain\'s Terrorism Act of 2000. Miranda, who was traveling from Berlin to Rio de Janeiro, where he lives with Greenwald, said he was threatened with prison if he didn\'t cooperate. Greenwald reported on U.S. and British surveillance programs for The Guardian based on leaks from former National Security Agency contractor Edward Snowden, who was granted temporary asylum in Russia. In a letter to the Home Office, Thorbjorn Jagland, head of the European human rights watchdog Council of Europe, expressed concern about Miranda\'s treatment, warning it \"may have a potentially chilling effect on journalists\' freedom of expression.\" Miranda\'s lawyers said nine items, including his laptop, mobile phone and DVDs, were seized during the detention. His lawyers said they began legal proceedings to \"protect the confidentiality of the sensitive journalistic material\" and that their motion was meant to secure a temporary injunction to prevent the authorities using the material. In a statement, the attorneys said, \"If interim relief is not granted then the claimant is likely to suffer irremediable prejudice, as are the other journalistic sources whose confidential information is contained in the material seized by the defendants.\" Brazilian Foreign Minister Antonio Patriota said his country would not retaliate against Britain over the incident but was awaiting an explanation. Patriota said he told British Foreign Secretary William Hague in a telephone conversation holding Miranda was \"counterproductive and does not contribute to an international coordinated action,\" the BBC said. Home Secretary Theresa May has defended police use of anti-terrorism laws to hold and question Miranda, saying such action \"was right\" if police believed Miranda had information useful to terrorists.