Police on Tuesday arrested several protest leaders ahead of today's anti-government rally in Bangkok, including Jatupat "Pai Daodin" Boonpatararaksa.
Police also plan to stop protesters from marching to Government House after a group of royalists brought supporters along the route of a royal motorcade.
A group of about 20 protesters, led by Mr Jatupat, set up tents on Tuesday in front of a McDonald's restaurant close to the Democracy Monument in Ratchadamnoen Avenue ahead of Wednesday's mass rally.
Mr Jatupat, who was speaking from the back of a pickup truck, had been giving a speech through a loudspeaker, next to a banner that said: "Dissolve the House under new rules and revoke Section 112". Section 112 is better known as the lese majeste law.
Police asked them to stop because Ratchadamnoen Avenue was due to be used by His Majesty the King's motorcade en route to the Grand Palace, where royal ceremonies were held on Tuesday to mark the fourth anniversary of the passing of His Majesty King Bhumibol Adulyadej The Great.
However, the demonstrators ignored the police request, prompting officers to arrest Mr Jatupat and other protest leaders.
Police said about 19 protest co-leaders were arrested for breaking the public assembly law by failing to secure prior permission from the police.
Police would on Wednesday prevent protesters from reaching Government House and would ask them not to use Ratchadamnoen Avenue, since it would be used by the royal motorcade.
