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The demonstrators, from the United Front for Democracy against Dictatorship (UDD), accuse the current government of being a puppet of the military.
Protest leaders are calling Wednesday their "D-Day" in their efforts to push Mr Abhisit to dissolve his four-month-old government and hold elections.
There is a strong sense of co-ordination to the reds' movement now - they were seen as being far less organised or resourced than the yellow-shirted demonstrators from last year.
All spoke with passion about why they had come and what they wanted - the prime minister to resign and elections to be held.
Pollawat Srichakkote travelled for five hours by road from Surin in the north-east of the country and said he had come to call for democracy in a Thailand divided between rich and poor.
"I want a more perfect democracy that is accepted by everyone in the world and for us to move forward," he said.
"The rich always do things to benefit themselves - it doesn't go to the brothers and sisters in society."