On Sunday, a bomb secretly attached to a police pickup truck exploded as it arrived at a checkpoint in Yala province, injuring two police officers and a woman food vendor, police Lieutenant Somporn Ritirak said.
He said the bomb was probably triggered remotely by a mobile phone.
Separately, two soldiers patrolling by motorcycle in Narathiwat province were badly hurt when a bomb planted by the roadside was detonated as they passed by, police Major Chalong Rattanapakdee said.
About 900 people have died in drive-by shootings, bombings and other violence in the southernmost provinces of Yala, Narathiwat and Pattani since January last year, when Muslim separatists reignited a decades-old but dormant insurgency.
King Bhumibol Adulyadej signed the decree into law on Sunday, authorising Thaksin to unilaterally declare a state of emergency and implement security measures, Cabinet Secretary Bowornsak Uwanno said.
The decree gives Thaksin the power to impose curfews, ban public gatherings, censor news, close publications, limit travel, detain suspects without charge, confiscate property and tap telephones, among other measures.
The sweeping powers - unparalleled for a democratically elected Thai leader - have been met with criticism from academics, the press and lawmakers, who say the decree is unconstitutional and dictatorial.
The Nation newspaper reported on Sunday that several senators have called for a joint parliamentary session to debate it.
"We believe Prime Minister Thaksin Shinawatra has crossed the line and stands to violate the civil rights of the people"
Senator Seri Suwan-phanond
"We believe Prime Minister Thaksin Shinawatra has crossed the line and stands to violate the civil rights of the people," the newspaper quoted Senator Seri Suwan-phanond as saying.
Thaksin on Saturday defended the decree, saying its powers would be used only if absolutely necessary.
The decree was issued after a daring raid on Thursday night by suspected Islamic insurgents on the capital of Yala province.
They destroyed electrical transformers to black out the city, then set off firebombs and other explosives in commercial areas and fired at security forces before escaping.
Thaksin visited Yala on Saturday "to give moral support to the authorities" there.