Two complaints have been filed with police against leaders of the weekend protest rally at Sanam Luang by the United Front of Thammasat and Demonstration.

One is an accusation of lese majeste under Section 112 of the Criminal Code, and the other over the installaton of a plaque symbolising democracy, allegedly in violation of the law on ancient monuments, antiques, objects of art and national museums, which applies to Sanam Luang.

About 1pm on Monday, Tul Sithisomwong, leader of the self-styled Multi-Coloured Group, filed a complaint with Chana Songkhram police against three protest leaders - Panusaya "Rung" Sithijirawattanakul, Arnon Nampa and Parit "Penguin" Chiwarak.

In the petition, the three were accused of violating Section 112 of the Criminal Code, or the lese majeste law, by delivering speeches calling for the reform of the monarchy and using inappropriate words, which allegedly caused Thai people in general to feel uncomfortable.

His petition was accompanied by photos of the event and audio recordings of the speeches made by the three protest leaders.

"I don't want them to end up in prison and, even so, I believe there would be a request for them to be pardoned. But I don't want to see this happen again," Dr Tul said.

"I don't mind if they talked about politics, the prime minister or the constitution, because they have the right to do so, but not about the monarchy, in their speeches over the media," he said.

Police accepted the complaint for consideration.

Earlier in the day, about 10.30am, Sathaporn Thiangtham, director of the Archaeology Division and representing the Fine Arts Department, filed a complaint with Chana Songkhram police against leaders of the protest for installing a plaque symbolising democracy in the ground at Sanam Luang.

Mr Sathaporn said doing so without permission may have violated the Ancient Monuments, Antiques, Objects of Art and National Museums Act of 1961, which applies to Sanam Luang. The ground is registered as an ancient site.

The complaint was accompanied by photos and related documents, and was also accepted for consideration.

The students' plaque declared that Thailand belongs to the people. It was dug up and removed on Sunday night, but no one has admitted seeing who did it.