Deputy Prime Minister Surapong Tovichakchaikul claimed that the video showed the explosive device being dropped in the road by a man who was walking in the march, and who then walked smartly away before it exploded.
Police say they are seeking two men, one of whom they believe they have identified from the video as being an aide to a former Democrat MP.
Pol Maj Gen Adul Narongsak, deputy chief of Metropolitan Police Bureau, said the video showed a man wearing a white hat walking near a pickup and dropping an object, before rushing to take shelter.
Police have identified the grenade as a Russian-made RGD-5, not used by any of Thailand’s armed forces.
Meanwhile, Tipayao Choojan, Mr Prakong’s wife, was consoled by protest leader Suthep Thaugsuban, and marched with him through the streets of Bangkok.
“I wanted to come to Bangkok [with my husband] for the rally but I had to stay in Patong to take care of my children,’’ she told BlueSky TV during the march headed by People’s Democratic Reform Committee (PDRC) leader Suthep Thaugsuban.
“He told me that he would not come back to Patong until the protesters won.”
She rushed to Bangkok after her husband was injured in the blast and stayed by his side until the end.
Mrs Tipayao met and hugged by the family’s idol, Mr Suthep, yesterday. She was also consoled by PDRC co-leader Putthipong Punnakan who gave her and her family B1 million on behalf of the PDRC.
Mr Prakong’s death has increased political tensions and has also raised questions over police involvement in the attack; war weapons, two police badges and a police cap bearing an “attacking unit’’ logo were allegedly found in a building near the bomb scene on Banthat Thong Rd.
During an address on the Pathumwan rally stage on Friday night, the PDRC leader pointed the finger at police in Narathiwat, claiming the insignia on the cap is that of a unit in the province.
However, Pol Maj Gen Pattanawut Angkanawin, chief of the Narathiwat police, yesterday denied the accusation, arguing that the cap and logo are different from those used in the southern province.
Police in Narathiwat have not been sent to join other units drafted in to help control the protests in Bangkok, he told a press conference, during which he showed media the cap worn by his units.
More than 2,000 protesters, led by Puttipong Punnakan and Sakoltee Patthyakul, stormed the National Police Office compound yesterday and submitted a letter demanding police find Mr Prakong’s killer.
Ms Yingluck yesterday denounced the Banthat Thong Rd grenade attack. She said she opposed all forms of violence and has directed police to find the attackers and take stringent legal action against them.
She insisted that her government had nothing to do with the blast and was not trying to incite unrest as claimed by the PDRC.
The Pheu Thai Party called on people to respond to the protest attack in a fair and even-handed manner, rather than simply blaming the caretaker prime minister for the incident.
Pheu Thai spokesman Prompong Nopparit said he suspected that the bombing had been a “set-up”.
He questioned why demonstrators changed their route at the last minute on Friday and then refused to let police collect evidence from the building thought to have been where the explosive device was thrown from.
The PDRC said on Friday that evidence found in a deserted building thought to have been the hiding place of the grenade suspect had been handed over to Pathumwan Police Station.
It added that the evidence was retrieved by its guards who refused to let police examine the scene, for fear they might destroy vital forensic information.
Evidence handed over to police by the PDRC included three disassembled M16 rifles, four walkie-talkies and a cap bearing an “attacking unit” logo, according to Mr Suthep.
Pol Lt Gen Kamrop Panyakaew, chief of the forensic science police office, said yesterday that the war weapons found in the room are fake.
Police said they had questioned the owner of the unoccupied building, which is due to be torn down. The owner admitted that the walkie-talkies and cap belonged to him, Gen Kamrop said, but denied ownership of the guns.
Mr Prakong is the ninth person to be killed in rallies to unseat caretaker PM Yingluck Shinawatra, according to the Erawan Centre, which is the Bangkok emergency medical service unit. Including 39 people injured in the Banthat Thong Rd attack, the toll of injured since October 31 is now 521, the centre said.