The attack killed 20 people in the Thai capital earlier this month.
Police are questioning the two men who they suspect were involved in Monday's deadly bombing.
Surveillance footage shows a young suspect carrying a backpack near the bomb scene on Monday.
At least 19 people died from the explosion at a major shopping center in Thailand's capital.
The decree allows security agencies to impose curfews, detain suspects, censor media, ban political gatherings of more than five people and declare areas off-limits