Twin explosions rocked "Bangkok Shutdown," a 12,000 protester-strong week-long demonstration flooding Bangkok's streets.
One explosion injured thirty-six people and the other explosion caused only smoke.
Friday's attacks were the first to occur during daylight this week.
At 1:04 p.m. local time, an explosive device suspected to be an M26 grenade exploded at an intersection close to where protest leader Suthep Thaugsuban was leading the march.
When Bangkok police arrived at the scene, they were booed by protesters and prevented by protest guards from searching for the bomb suspect, with protest guards citing concern that police would tamper with any evidence. Security guards and soldiers discovered a room in a deserted building nearby that local television reported contained loaded weapons and walkie-talkies.
A Bangkok Post reporter reported four walkie-talkies were recovered as well as two police badges.
A second explosion at another rally site caused significant smoke and was likely set off by a large firecracker.
Evening attacks in previous days sometimes involved M26 grenades, with grenades thrown at a Democrat Party member's house Wednesday night and at the Bangkok governor's house on Thursday evening. On Tuesday night, two protesters were shot, buses were burned, police officers assaulted, and an explosive device was thrown at another opposition leader's house.
Anti-government protests, led by the opposition Democrat Party, have been ongoing since November. Protesters are calling for the removal of the Prime Minister Yingluck Shinawatra, whom they accuse of acting on behalf of her brother, ousted former Prime Minister Thaksin Shinawatra.
The "Bangkok Shutdown" was organized by the People's Democratic Reform Committee. 170,000 rallied Monday, with numbers decreasing through the course of the week.