Sunday's anti-government rallies are likely to cause heavy disruption to traffic at Ratchaprasong and Lat Prao intersections, two of the city's main junctions, said the Metropolitan Police Bureau (MPB).
Police are concerned the rallies would halt traffic in the area today after organisers outlined their "car park" protest strategy.
Convoys of protesters' vehicles are to meet at the Ratchaprasong intersection, the Democracy Monument and Ayutthaya at 2pm before setting off an hour later.
At 6pm, the protesters will honk their horns before dispersing.
Nattawut Saikuar, co-leader of the red-shirt United Front for Democracy against Dictatorship (UDD), will lead the protest movement on Sunday.
On Saturday, MPB deputy commissioner Jirasan Kaewsang-ek said heavy traffic congestion is likely to be triggered by the protests. Police will be on hand to direct traffic and recommend alternative routes for motorists.
Protests in the past week affected traffic as rally participants did not stay in one place but moved to various locations.
He said some protesters vandalised and damaged traffic lights and torched state property, including police kiosks as well as police rescue vehicles.
Pol Maj Gen Jirasan said police were also in danger of being harmed during the clashes with protesters, which made it difficult for them to provide assistance to people in distress.
The MPB has asked the City Hall to repair damaged traffic lights and closed-circuit cameras. The protesters will be sued for damages which are being compiled.
Also on Saturday, Pol Col Kissana Phatthanacharoen, deputy spokesman for the Royal Thai Police Office, said police had acted within the law in launching crowd-control measures during the clashes with the protesters in Bangkok on Friday.
He said police warned the protesters to stop the rally but they refused to comply. Some attacked the police who countered them using anti-riot equipment.
Meanwhile, the Expressway Authority of Thailand has confirmed the expressway structure was sound even though a section was exposed to a fire which engulfed a police kiosk underneath during anti-government protests in Din Daeng on Aug 10.
