The members of detained Aung San Suu Kyi’s party, all aged between 22 and 40, were sentenced late last month, Nyan Win, a spokesman for the National League for Democracy party, told AFP.
They were arrested for urging people to vote “No” in a nationwide constitutional referendum, which was held and passed by the ruling junta in May, their party said.
“We will appeal against the conviction next week. We will try to do whatever we can do for them,” Than Pe, a senior NLD member, told AFP.
Nyan Win said another senior NLD member had been arrested on Wednesday without reason.
It was unclear if the arrest related to a small bomb which exploded at the offices of the pro-government Union Solidarity and Development Association (USDA) early Tuesday morning.
A further 14 people arrested on June 19, Aung San Suu Kyi’s birthday, were still in detention, Nyan Win added.
Myanmar has been ruled by the military junta since 1962.
They claimed May’s referendum, carried out in the wake of a devastating cyclone which left 138,000 dead or missing, had a 98 percent turnout with more than 92 percent endorsing their charter.
Aung San Suu Kyi led the NLD to a landslide victory in 1990 elections but has never been allowed to govern.