
The explosion occurred at 4:20 am in front of the Santi Maitree Building about 50 metres away from the rally stage of the PAD.
Methee Uthong, 24, suffered head injuries and was rushed to the Vajira Hospital. Doctors said the guard was now safe.
The sound of the explosion caused panic among the demonstrators.
The speaker on the stage told the crowd to remain calm and said PAD guards were now closing on certain infiltrators.
Nittaya Nuttrakian, 49, a housewife who joined the rally, said she had never expected that any incident to happen inside the Government House complex. She said she was now scarred.
A PAD guard, who identified himself as Lek, said he thought an assailant might hurl the bomb over the fence into the compound.
Chamlong Srimuang and Somsak Kosaisuk, two co-leaders of the PAD, held a press conference and held the government responsible for the bombing.
The two said the PAD did not stage the explosion to try to draw public support. They denied that the PAD had internal conflicts, which led to the explosion.
Somsak said he believed the government sent an infiltrator to plant and detonate the bomb to scare away supporters of the PAD.
Chamlong said the PAD would step up security because it had the highest concern over the life of demonstrators.