The move has drawn criticism from the US which condemned the decision as a rejection of efforts towards national reconciliation and democracy in the country.
Aung Thein, a lawyer, said on Tuesday it was not clear what statement the activists have been accused of making.
He said he only learnt of the charges from relatives of the activists, who found out while visiting them in prison last week.
"They are likely to face trials behind closed doors inside Insein prison," he told AFP. "I am ready to defend them, but so far I have not been allowed to meet with them."
Tom Casey, a US state department spokesman, said the decision is "further evidence that the regime is rejecting all efforts to promote dialogue and national reconciliation".
"The United States calls on [Myanmar leader] Than Shwe and his regime to release all political prisoners, including Aung San Suu Kyi," he said.
He added that Washington wanted the military to "cease the arrests and prosecution of democracy and human rights activists and to begin a meaningful and time-bound dialogue with Burma's democratic and ethnic minority representatives on a transition to a civilian, democratic government".
Last week London-based Amnesty International said 700 people were still behind bars following last year's demonstrations which grew into pro-democracy marches led by Buddhist monks.
The human rights group said more than 80 were unaccounted for and were probably "the victims of enforced disappearance".