Thai police fired water cannon, tear gas and rubber bullets on Sunday (July 18) to stop more than 1,000 people marching on the office of the prime minister.
These anti-government protesters are calling for Prayuth Chan-ocha to resign.
They accuse him and his government of mismanaging the COVID-19 pandemic.
The demonstration also comes after the Thai government tightened restrictions.
On Friday (July 16) it imposed a nationwide ban on public gatherings of more than five people.
The maximum penalties are two years in prison, a 40,000 baht fine or both.
Protester Ittinan says he's aware of the risks.
"I understand that the situation is not getting any better but we have to come out and show them that we are not happy about the measures imposed by the government. It's like they only wanted everything to stop at a standstill but they were not trying to fix anything. Yes, I know that we are risking getting infected, that's why I try to protect myself by wearing gloves and a gas mask so that I would be at low risk."
Police used force after some protesters tried to dismantle barbed wire and metal barricades leading to Government House, where the prime minister works.
The protest marked one year since the first of a wave of large-scale street demonstrations, led by youth groups, that attracted hundreds of thousands of people across the country.
Thailand reported 11,397 infections and 101 deaths on Sunday - bringing its cumulative total to more than 403,000 cases and 3,341 fatalities.
The vast majority of them are from an outbreak since early April which is fuelled by the highly transmissable Alpha and Delta variants.