The explosion occurred at 5:20 am.
The man was rushed to the intensive care unit of the Chulalongkorn Hospital.
Channel 3 reported that the explosion occurred near a tent of security guards of the PDRC or about 100 metres from the rally stage. It said a guard was severely injured.
The centre said an explosion also occurred at the Lotus Chaengwattana at 10:30 pm Monday. No casualty was reported.