Anti-government protesters attacked a car carrying Thai Prime Minister Abhisit Vejjajiva Tuesday as political tensions erupted in violence.
The clash was the most serious since supporters of fugitive former prime minister Thaksin Shinawatra began surrounding Mr. Abhisit's offices late last month.
Supporters of Mr. Thaksin say they expect hundreds of thousands to show up in Bangkok Wednesday in their latest bid to force the current prime minister to step down.
The protesters want Mr. Abhisit to dissolve his four-month old government and call snap elections.
Mr. Abhisit says he must stay in office and has vowed to prevent opposition protests later this week from escalating into what he called "civil war."
During Wednesday's rally, protesters plan to march from Government House to the home of King Bhumibol Adulyadej's top adviser - Prem Tinsulanonda.
Protesters accuse Prem of masterminding the 2006 coup that ousted Mr. Thaksin from office.
Mr. Abhisit assured the nation during a televised address Monday that his government is prepared to deal with Wednesday's planned demonstrations.
Mr. Abhisit said security forces are capable of preventing unrest under normal laws and without the declaration of a state of emergency.
Last Friday, Thaksin rejected a government offer to hold talks to end the political turmoil that has plagued Mr. Abhisit's four-month-old government.
Thaksin has been delivering almost daily speeches to supporters in Bangkok through a video link from an undisclosed location abroad. He fled the country last year before a Thai court sentenced him to two years in prison for graft. He denies any wrongdoing and is believed to be living mainly in the Middle Eastern emirate of Dubai.