Chaos erupted in Bangkok's Ramkamhaeng area, where protesters armed with sticks attacked a bus and taxi and badly beat two people, police and witnesses told Reuters.
Anti-government protesters attacked motorcyclists and vehicles near a stadium rally by supporters of Prime Minister Yingluck Shinawatra.
About 1,000 anti-Government protesters have broken into the compound of the Royal Thai Army headquarters, according to Reuters.
It marks the latest escalation in a demonstration seeking to topple Prime Minister Yingluck Shinawatra.
Thailand's Prime Minister Yingluck Shinawatra has warned that the occupation of government buildings by protesters threatens the stability of the country, but said she would no use force against them.
Anti-government protesters forced their way inside the Finance Ministry and Foreign Ministry today in a bid to overthrow Shinawatra.
They accuse her of being a puppet for her brother, former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 military coup and convicted two years later of graft - charges he denies.
Staff sought to protect the reputation of Thai Airways this morning by blurring out its logo on the side of an aircraft following an emergency landing.
A Thai Airways Airbus 330-300 skidded off the runway while landing at Bangkok's main airport after its landing gear malfunctioned, the airline said. The flight from Guangzhou, China, was carrying 288 passengers and 14 crew members.
Thirteen people were injured while evacuating the plane. Following the incident, workers, using a crane, blacked out the Thai Airways logo on the tail and body of the aircraft.
Thai Airways official Smud Poom-On said that "blurring the logo" after an accident was a recommendation from Star Alliance known as the "crisis communication rule," meant to protect the image of both the airline and other members of Star Alliance.
The incident occurred less than two weeks after 20 passengers were injured when a Thai Airways Airbus A380 hit severe turbulence as it was descending to Hong Kong's airport.