BANGKOK, Thailand (CNN) -- Anti-government demonstrators say they will gather in the thousands in the capital city Wednesday in a bid to force Thailand's prime minister from office.
"Red Shirt" protesters stage a demonstration on Tuesday.
The massive demonstration is the culmination of a movement that has intensified in recent weeks to destabilize the government of Prime Minister Abhisit Vejjajiva.
On Tuesday, the "Red Shirt" protesters -- named for the color of their shirts -- attacked Abhisit's motorcade as it was struck in traffic. He escaped unharmed.
Abhisit was leaving a Cabinet meeting in the beach resort city of Pattaya when the protesters pelted plastic water bottles and sticks at the car, a government official said.
The prime minister's security detail shielded him and moved him to another vehicle. Television footage showed protesters unsuccessfully trying to grab the driver of the second car. They also smashed the second vehicle's rear window.
The Red Shirt protesters have declared Wednesday their D-Day, claiming they will draw up to 300,000 people from the eastern and northern part of the country.
Those two regions are a stronghold of former prime minister Thaksin Shinawatra, to whom the Red Shirts owe their allegiance.
Thaksin was deposed from power in a coup in 2006, but still wields considerable clout. A party loyal to him, the People Power Party, assumed office in 2007 elections.
Those opposed to Thaksin took to the streets, dressed in yellow shirts and earning the moniker, the "Yellow Shirts."
They staged sit-ins for more than two months last year -- occupying the headquarters of the government and blockading Bangkok's major international airport.
The demonstrations ended December 2 when a court ruled the People Power Party was guilty of electoral fraud.
The move paved the way for Abhisit to assume the prime ministership.
Now, the tables are turned -- with Thaksin supporters demonstrating.
They maintain Abhisit's government was not democratically elected. They want him to leave office and schedule new elections.