Although pro-government "Red Shirt" protesters have largely been drowned out in Bangkok in support of Prime Minister Yingluck Shinawatra, leaders said they would step in if the government loses control to arch-rival Suthep Thaugsuban.
Suthep's followers continue to occupy key locations in the city, including Government House and the Finance Ministry, the Wall Street Journal reported Friday.
"If there's a coup or if Suthep gains power, we will fight back," said Tida Tawornseth of the United Front of Democracy Against Dictatorship, the largest group supporting Yingluck and her brother, former Prime Minister Thaksin Shinawatra.
Suthep's supporters have called for a rally Monday in Bangkok, and the Red Shirts plan a separate demonstration Tuesday in Ayuttaya, an hour's drive from the capitol.
The anti-government protesters still occupy three main rally sites, although their numbers dwindled from tens of thousands to a few thousand Friday, the newspaper said.
"We have to end this fight by Monday," Suthep said Friday. "A large number of people must come out to defeat the government in this non-violent protest. It's time for Thai people to take sides."