Thailand's most popular politician was banned from politics for 10 years and his party dissolved on Wednesday over his attempt to reform archaic royal defamation laws.
The Constitutional Court in Bangkok voted "unanimously" to dissolve the Move Forward Party (MFP) and ban its executive board, which includes its former leader Pita Limjaroenrat, for 10 years, judge Punya Udchachon said.
Pita, 43, led the reformist MFP to a shock first place in a general election last year after striking a chord with young and urban voters with his pledge to reform Thailand's strict royal defamation law.
"Let's be sad today for one day but tomorrow we will move on and let's release the frustration through the next ballot we will cast in the next election," Pita told a news conference on Wednesday evening.
His bid to become prime minister was blocked by conservative forces in the Senate. A coalition of army-linked parties took office instead under Prime Minister Srettha Thavisin.
The European Union, the United States and rights groups blasted the court's decision, which the EU said harmed democratic openness in Thailand.
"No democratic system can function without a plurality of parties and candidates," an EU spokesperson said in a statement.
US State Department spokesman Matthew Miller said the ruling ran "counter to the aspirations of the Thai people for a strong, democratic future", according to a statement.
Amnesty International described the decision as "untenable" and said it showed that laws in Thailand were being used to intimidate critics, while the Asian Forum for Human Rights said it posed "serious risks to democratic principles".
Pita's political career was already shaken in March when Thailand's election commission asked the top court to dissolve the MFP.
That followed a ruling that the party's pledge to reform the lese-majeste law amounted to an attempt to overthrow the constitutional monarchy.
Lese-majeste charges are extremely serious in Thailand, where King Maha Vajiralongkorn enjoys a quasi-divine status that places him above politics.
Dozens of supporters dressed in the party's trademark orange gathered in front of MFP headquarters in Bangkok.
Siriporn Tanapitiporn, a 53-year-old food market vendor, cried after the verdict was read out.
"But I have faith in the younger generation, they will return the democracy back to our country," she said.
Sakhorn Kamtalang, 60, said the court didn't have the right to dissolve the party.
"To me, Pita is my PM. The current PM is just a salesman, who isn't fit as the country leader," she said.