At the same time, Washington distanced itself from the incident.
"It's an extraordinary twist to a complicated story," Keith Vaz, chairman of Parliament's Home Affairs Select Committee, told the BBC a day after Brazilian citizen David Miranda, the partner of U.S. journalist Glenn Greenwald, who lives in Brazil, was held at London Heathrow Airport.
Miranda was held for 9 hours, the maximum allowed by law, before being released without charge.
He said Monday all of his electronic equipment, including his laptop computer and cellphone, were confiscated.
Miranda was traveling home to Rio de Janeiro from Berlin and had a stopover in London. In Berlin, he met with Laura Poitras, a U.S. filmmaker who has worked with Greenwald on the Snowden leaks about secret U.S. and British surveillance programs.
Greenwald writes for the British newspaper The Guardian.
Brazil said Miranda's detention was "without justification" and summoned British Ambassador Alex Ellis for an explanation. Ellis did not acknowledge any errors by London and did not apologize for the detention, Brazil's External Relations Ministry said in a statement.
Brazilian Foreign Minister Antonio Patriota also phoned British Foreign Secretary William Hague to express Brazil's condemnation of the action.
"What needs to happen pretty rapidly is, we need to establish the full facts," Vaz, an opposition Labor Party lawmaker, told the BBC Monday.
"Now you have a complaint from Mr. Greenwald and the Brazilian government -- they indeed have said they are concerned at the use of terrorism legislation for something that does not appear to relate to terrorism. So it needs to be clarified, and clarified quickly," said Vaz, one of several British politicians demanding an explanation of why anti-terrorism laws were used to detain Miranda.
A spokesman for Britain's Home Office, responsible for immigration, security and law enforcement, declined to comment Monday, saying the detention was an "operational matter for the police."
London's Metropolitan Police Service, also known as Scotland Yard, said it lawfully held Miranda, 28, under Schedule 7 of Britain's Terrorism Act 2000. The section lets authorities stop and question people at the country's borders to determine if they're involved in terrorism.
In Washington, White House spokesman Josh Earnest told reporters London gave the administration advance notice it intended to detain Miranda when his plane landed.
But he distanced the administration from the incident, saying it made no request for London to hold him.
"This is the British government making a decision based on British law, on British soil, about a British law enforcement action," he said.
"They gave us a heads-up, and this is something that they did not do at our direction and it's not something that we were involved with. This is a decision that they made on their own."
Earnest declined to criticize the detention or say if President Obama was pleased with it. He also would not rule out that London gave Washington material taken from Miranda.
Miranda arrived in Rio de Janeiro Monday and was greeted by Greenwald, who said he planned, in response to the detention, "to write much more aggressively than before" about government snooping.
He also said he had plenty of copies of all documents.