Activist Panusaya "Rung" Sithijirawattanakul on Wednesday denied any attempt to overthrow the monarchy in demands made and other actions during anti-government rallies.
"They said I am trying to overthrow (the monarchy) when I am only asking for reform," she wrote a short Twitter message after the Constitutional Court's ruling on her case.
The court ruled Ms Panusaya, who is a Thammasat University student, human rights lawyer Arnon Nampa and activist Panupong "Mike" Jadnok aimed at overthrowing the democratic institution with the King as head of state in their speeches during a rally at Thammasat University Rangsit campus on Aug 10 last year and at other events afterwards.
Their actions violated Section 49 of the constitution, the court ruled.
The Twitter hashtag #reformnotabolishment reacted strongly after the judges delivered their ruling.
The United Front of Thammasat and Demonstration issued a statement refusing to accept the ruling of the court, saying the judges refused to allow the three protesters to present additional evidence in their defence.
The group also said they had no intention to overthrow the constitutional monarchy.
Supporters of the three activists waiting outside the court premises set fire to a model of the Democracy Monument in protest at the ruling.
The court ruled on Wednesday on a petition filed for consideration by Natthaporn Toprayoon, a former adviser to the Ombudsman, in September last year.
"The ruling today is a starter, that peace will finally be returned to society," Mr Natthaporn said after the ruling, which also bans all activities threatening the highest institution.
He said the court's decision would lead to the Election Commission deciding whether to move for the disbandment of the Move Forward Party. Mr Natthaporn claims the party supported the protests.