The detentions came a day after the 10th anniversary of a nationwide uprising against military rule - which ended in a bloodbath - passed calmly without reports of disruption anywhere in the country.
Those held include six US citizens, three Thais, three Malaysians, three Indonesians, two Philippines nationals and one Australian, a government official said in a statement.
The ad hoc coalition of pro-democracy organisations that sent the activists released the name only of Jaran Dithachai, a political science professor at Bangkok's Rangsit University, pending notification of the families of the others held.
Annelyn Deluna, a coalition member, said that all the activists sent to Myanmar had been arrested. They were handing out "goodwill messages" supporting human rights and democracy and urging people to remember the uprising 10 years ago.
"We thought there were six who reached the airport, but it seems that the last time they were seen they were being brought to an office in the airport," Deluna said. None appeared to have been aboard the day's last Thai Airways flight from Myanmar, Deluna said. She was hopeful they would be deported Monday.
The government statement said that 18 foreigners "were apprehended attempting to incite unrest in Yangon."
"Foreigners who were distributing the pamphlets in downtown Yangon were reported to the police by citizens and taken to police stations for questioning and are being detained while the investigation continues," the statement added.Reuse content