Protesters under the Network of Students and People for Reform of Thailand (STR) started marching to the stadium in Din Daeng area at 6am.
They tried to break into the stadium, cutting the chain that locked the entrance gate. Shortly before 8am, police fired tear gas and rubber bullets at the protesters, according to reports in Thai media.
Protesters did not prepare equipment to protect themselves from the tear gas and rubber bullets. Many of them were injured.
The reports say the wind also blew the tear gas towards nearby Pibul Prachasan School where there are more than 1,000 student. The school advised parents to take their children home.