More students are marching to unseat Prime Minister Samak, adding to an ideologically mixed coalition of businesspeople, royalists, and academics.
Activists have disrupted air and rail travel, and refuse to leave government grounds.
Former Prime Minister Thaksin Shinawatra, facing corruption charges, fled to England this week. His absence may hurt the pro-Thaksin ruling party but also ease political strife.
Weeklong antigovernment rallies have raised concerns of a second coup in two years.
At issue are clauses that could shut down the popular former prime minister's party.
A May 10 poll could underscore how unpopular the regime is, as it slowly opens to foreign aid.
Thailand boomed on exports to the US. Now it's looking at populist incentives to stimulate growth.
The ousted Thai prime minister, who returned Thursday, faces corruption charges and a divided nation.
The majority coalition is expected to name a controversial prime minister.