World: Thai Prime Minister Resigns

Until then, Deputy Prime Minister Chaowarat Chandeerakul will become the caretaker prime minister, said Suparak Nakboonnam, a government spokeswoman. She said parliament will have to pick a new prime minister within 30 days.

Despite the appearance of a smooth political transition, the ruling is expected to widen the dangerous rift in Thai society that many fear could lead to violence between pro- and anti-government groups.

Late Monday, an explosive device fired from an elevated highway fell among hundreds of protesters inside Don Muang airport, killing one person and wounding 22. The death raised to seven the number of people killed in bomb attacks, clashes with police and street battles between government opponents and supporters.

The protesters accuse Somchai of being a proxy of ousted Prime Minister Thaksin Shinawatra, the alliance's original target. Thaksin, who is Somchai's brother-in-law, was deposed in a 2006 military coup and has fled the country to escape corruption charges.

Alliance supporters are largely middle-class citizens who say Thailand's electoral system is susceptible to vote-buying and argue that the rural majority -- the Thaksin camp's political base -- is not sophisticated enough to cast ballots responsibly.

They have proposed discarding direct elections in favor of appointing most legislators, fostering resentment among rural voters.