Antigovernment activists plan to defy a tough security law to rally Saturday on the third anniversary of a military coup.
Attack on Sondhi Limthongkul could further weaken the economy and strengthen hard-liners
The prime minister Friday extended the state of emergency he declared last week after rival protesters descended on the capital.
The antigovernment group swarmed the airport where the prime minister was supposed to arrive Wednesday.
More students are marching to unseat Prime Minister Samak, adding to an ideologically mixed coalition of businesspeople, royalists, and academics.