Thousands of people protested in the Thai capital Bangkok on Saturday against the government led by Yingluck Shinawatra.
A new royalist group behind the demonstration accuses her of being a puppet of her brother, former prime minister Thaksin Shinawatra, who was deposed six years ago.
Pitak Siam is a relatively new name in Thai politics, but is supported by "yellow shirt" members of the People's Alliance for Democracy who helped destabilise governments either led or supported by Mr Shinawatra in 2006 and 2008.
Broadly speaking, they want the government of Yingluck Shinawatra removed from office, saying it is corrupt and controlled by her fugitive brother.
The leader of the group denied wanting to engineer a coup, but told the BBC that Thailand at present is a fake democracy.
Before the protest, anti-riot police wielding plastic shields fired gas canisters at demonstrators who tried to climb over cement and barbed wire barriers blocking entry to the rally site at Royal Plaza.
At least seven police were wounded and up to 132 protesters were arrested in the clash near the United Nations headquarters in Bangkok.
Authorities expected tens of thousands of people at the demonstration, the first major street protest against Ms Shinawatra's 16-month-old administration, AFP reports.
The government has invoked a special security law, the Internal Security Act, in three districts of the capital to cope with any unrest.
Mr Yingluck on Thursday voiced fears that the protesters aimed to use violence and to "overthrow an elected government and democratic rule".