It was not immediately clear what caused the blasts that hit several spots in an area packed with heavily armed troops and studded with banks, office towers and hotels.

Preliminary investigation showed that the blasts were caused by M-79 grenades, the army spokesman said.

"They all were M-79s," said Col. Sansern Kaewkamnerd told Reuters. "There is no report of deaths," he added.

Hundreds of fully armed troops have poured into the area since Monday to keep anti-government protesters at a nearby rally site from marching into the business district.