Thanathorn Juangroongruangkit was charged with lese majeste, Thailand's strict royal defamation laws over a Facebook video in which he questioned the country's over-reliance on coronavirus vaccines from a company linked to the royal family.
The elections in 76 Thai provinces are the first since Prime Minister Prayut Chan-ocha overthrew an elected government in a military coup.
Thousands of anti-government protesters in Thailand donned sports gear in the pre-dawn darkness for the biggest political protest in the country in years.
Demonstrations come after Thailand's election panel asked Constitutional Court to dissolve the opposition Future Forward Party, accusing it of infringing laws by accepting multimillion-dollar loans from its leader.
Thailand's junta chief Prayuth Chan-ocha is the first civilian prime minister of the country since 2014.
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