RIO DE JANEIRO (Reuters) - The journalist who first published secrets leaked by fugitive former U.S. intelligence agency contractor Edward Snowden vowed on Monday to publish more documents and said Britain will “regret” detaining his partner for nine hours.

U.S. journalist Glenn Greenwald (L) walks with his partner David Miranda in Rio de Janeiro's International Airport August 19, 2013. REUTERS/Ricardo Moraes

British authorities used anti-terrorism laws on Sunday to detain David Miranda, partner of U.S. journalist Glenn Greenwald, as he passed through London’s Heathrow airport.

Miranda, 28, a Brazilian citizen, said he was questioned for nine hours before being released without charge, minus his laptop, cellphone and memory sticks, which were seized.

Greenwald, a columnist for Britain’s Guardian newspaper who is based in Brazil, said the detention was an attempt to intimidate him and stop him from publishing more secrets leaked by Snowden on U.S. surveillance of global internet communications.

Snowden, who has been granted asylum by Russia, gave Greenwald from 15,000 to 20,000 documents with details of the U.S. National Security Agency’s surveillance programs.

Asked by a reporter if the detention of his partner would deter him from future reporting, Greenwald said the opposite would happen.

“I will be far more aggressive in my reporting from now. I am going to publish many more documents. I am going to publish things on England, too. I have many documents on England’s spy system. I think they will be sorry for what they did,” Greenwald, speaking in Portuguese, told reporters at Rio de Janeiro’s airport where he met Miranda upon his return to Brazil.

Greenwald said in a subsequent email to Reuters that the Portuguese word “arrepender” should have been translated as “come to regret” not “be sorry for.”

“I was asked what the outcome would be for the UK, and I said they’d come to regret this because of the world reaction, how it made them look, and how it will embolden me - not that I would start publishing documents as punishment or revenge that I wouldn’t otherwise have published,” he said in the email.

Miranda told reporters that six British agents questioned him about all aspects of his life during his detention in a room at Heathrow airport.

Brazil complained about the “unjustified” detention of Miranda under a British law used for suspects of terrorism.

Brazilian Foreign Minister Antonio Patriota called his British counterpart, William Hague, on Monday to express his concern. They agreed their governments would remain in contact over the incident, Britain’s ambassador in Brasilia, Alex Ellis, said in a statement.

Many Brazilians are still upset with Britain’s anti-terrorism policies because of the death of Brazilian electrician Jean Charles de Menezes, who was mistaken for a suspect in a bombing attempt in 2005. Menezes was shot seven times in the head by police on board an underground train at a London station.

Greenwald met with Snowden in June in Hong Kong, from where he published the first of many reports that rattled the U.S. intelligence community by disclosing the extent of surveillance by the NSA on telephone and internet communications.

Snowden faces criminal charges in the United States after leaking documents disclosing the previously secret U.S. internet and telephone surveillance programs. Russia rejected American pleas to send Snowden back to the United States for trial, instead granting him a year’s asylum on August 1.

The White House said on Monday that U.S. officials did not ask the British government to question Greenwald’s partner, though British authorities did give their U.S. counterparts a “heads up” before detaining Miranda.

Brazil, whose president, Dilma Rousseff, is scheduled to make a state visit to Washington in October, declined to consider an asylum request from Snowden. But some politicians angered by the disclosure of NSA surveillance of internet communications of Brazilians proposed granting him asylum in Brazil.