Mr. Bini “is a highly respected expert in digital security and cryptography and is recognized for major contributions in the field,” Article 19 said in a statement. David Kaye, the United Nations’ special rapporteur on freedom of opinion and expression, said that nothing revealed so far connects Mr. Bini to any crime.

Danny O’Brien, the international director at the Electronic Frontier Foundation, said Mr. Bini’s work had helped build a safer internet. The evidence that Ecuadorean authorities presented was simply “the sort of equipment you have when you work in those sort of projects,” he said.

“People contributing their free time to protect internet users have their skills and their lifestyle used against them,” Mr. O’Brien said, a point of recurring concern among human rights advocates.

Mr. Bini was arrested at the international airport in Quito, Ecuador’s capital, on his way to Japan to take part in a martial arts program, said his girlfriend, Sofia Celi. Later, government officials went on television to applaud the arrest — and mention his visits to Mr. Assange.

The minister of interior, María Paula Romo, said the government was trying to prevent Ecuador from turning into a center for digital “espionage and piracy.” Ms. Romo said she could not comment on the evidence against Mr. Bini, but she, too, noted that he had visited Mr. Assange at the Ecuadorean Embassy “at least a dozen times.”

Mr. Bini’s lawyers say the detention is unlawful. They say he was denied access to lawyers for 17 hours, was not informed of the charges against him, and was not offered a translator, as required by local laws. His lawyers said they have been harassed and threatened by the police.