The red-shirts are attempting to block troops arriving into Bangkok. The protesters set up blockades in many of Thailand's north and northeastern provinces, and are vowing to take more aggressive measures.
Anti-government protesters stopped a military truck and a humvee heading into Bangkok on Monday night.
Their aim, to prevent more security troops from entering the capital.
More than 200 protesters are setting up informal checkpoints along Vipavadee Rangsit Road, a main avenue connecting Bangkok to northern provinces.
The protesters are scanning private vans and buses to make sure there are no troops inside.
On Monday the protesters quickly surrounded a humvee carrying four soldiers, and a truck with about a dozen more, preventing them from moving.
Some of the red-shirts rode with the soldiers to a police station, making sure the vehicles did not proceed to the city centre, where the red-shirts maintain a heavily fortified protest site.
Similar blockades have been set up in many of Thailand's north and north-eastern provinces.
The red-shirts, supporters of former premier Thaksin Shinawatra, have been seizing military cargo and vehicles.
In Pathum Thani, troops detained 20 protesters and chased away hundreds who were blocking the roads.
The leaders of the anti-government protesters called for resistance, saying a military crackdown was imminent.
The protesters, known for wearing their trademark red-shirts, have been instructed to don clothes of other colours to avoid arrest.
A continuing stalemate deepens the Thai political crisis, after Prime Minister Abhisit Vejjajiva recently rejected a compromise offer from the red-shirts.
The red-shirts responded by vowing more aggressive measures including laying siege to a high-end mall.
The protesters are back to their original demands of immediate dissolution of parliament and snap elections.