At least 21 protesters will be charged with defacing the Royal Thai Police Office (RTPO) building's sign on Wednesday.
The demonstrators, under the Network of Students and People for Reform of Thailand (NSPRT), removed the letters spelling out the RTPO building's name and spray-painted the words NSPRT, along with some offensive words, over it.
A police clean-up crew has painted the damaged name sign black, pending its restoration and replacement.
Police spokesman Piya Uthayo said CCTV footage shows at least 21 people were involved in defacing it.
The NSPRT is an ally of the People's Democratic Reform Committee (PDRC). The protesters will be charged with destroying state property, an offence that carries a 10-year jail sentence.
Police are not angry with the protesters, but they are obliged to take action, Pol Maj Gen Piya said.
National police chief Pol Gen Adul Saengsingkaew has assigned his deputy Gen Aek Angsananont and his assistant Pol Lt Gen Winai Thongsong to identify the suspects.
Pol Maj Gen Piya said the protesters hoped to provoke police into using force. It was a litmus test for police tolerance, he said.
The name sign was blessed by the late Supreme Patriarch in 2004, when the old Police Department became the Royal Thai Police, he said.
Pol Maj Gen Piya said the RTPO will not accept the NSPRT's offer to replace the damaged sign with a new one.
Pol Gen Pira Pumpichet, a senior adviser, will find the money to pay for the sign's restoration.
Meanwhile, about 300 protesters of the People's Democratic Force to Overthrow Thaksinism (Pefot), another ally of the PDRC, organised a rally in front of the Office of the Attorney-General (OAG) on Ratchadaphisek Road yesterday to demand that officials stop working.
The OAG has insisted on reopening today, despite the raid.
"Our duties cannot be suspended. We have to give legal aid to people," said OAG spokesman Nantasak Poonsuk, also director-general of the OAG's special cases office.
The OAG and Pefot yesterday held talks for an hour, which ended with the agreement to shut down the OAG yesterday.
Nantasak and Kamol Thamsereekul, director-general of the OAG's criminal cases office, told Pefot why the OAG has to keep working. Mr Nantasak said shutting down the OAG would affect court cases.
The protesters dispersed after the OAG agreed to send some of its officials back home.