"The government put pressure on us but we are staying put," said Nattawut Saikuar, one of the leaders of the red-shirted supporters of fugitive former premier Thaksin Shinawatra.
"Now we will retaliate with our own offensive."
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Riot police have surrounded tens of thousands of Red Shirts who have been rallying in the city's commercial heartland since Saturday, after the government declared it was illegal to gather there and on 11 other routes.
"We will resume our march on the 11 banned roads and break every checkpoint," Nattawut told the crowd.
"I'm asking for 5,000 volunteers to come on this mission to force the military back to the barracks," he said.
