The Ratsadon group has instructed supporters to gather at Democracy Monument at 3pm on Wednesday before marching to the Bureau for a protest over royal assets.
Calling the demonstration “very sensitive”, police spokesman Yingyot Thepjamnong said police would try to prevent conflict but would prosecute anyone who broke the law.
A total of 6,045 officers will be deployed to keep order and prevent damage to historical sites, he added.
Deputy Bangkok police chief Piya Tawichai said police would try to separate any royalist demonstrators from the anti-establishment protesters.
Nearby buildings such as Saint Gabriel’s College would be closed on Wednesday, he added.
Meanwhile, police charged pro-democracy activist Piyarat Chongthep with sedition on Tuesday in Ubon Ratchathani province, over a protest speech he gave in August.
Police are also reportedly preparing lese majeste charges against fellow activists Arnon Nampa, Parit “Penguin” Chiwarak and Panusaya “Rung” Sithijirawattanakul. The lese majeste law, which hasn’t been used for more than two years, carries a jail term of up to 15 years.
Police spokesman Krissana Pattanacharoen said on Tuesday that police had charged 170 people with protest-related offences, 100 in Bangkok.
He also defended the use of tear-gas chemicals against protesters, saying the substances were not lethal.