The suspects were identified as Wichan Rakchart, 34, Prakongsri Siriman, 39, Winas Prikpet, 34, Praditthong Chaipanha, 58, and Sombat Kaeosuk, 29. Weapons seized from them were shown at a Crime Suppression Division news conference on Friday.
The weapons included assault rifles, pistols, assorted ammunition and grenades.
Deputy national police chief Srivara Ransibrahmanakul said the weapons were en route to Bangkok for use during the rally in Bangkok on Tuesday, but security authorities uncovered the plot and prevented it happening. He said the plan was the work of hardcore red-shirt political activists.
Police seized 10,000 5.56mm bullets from a silver Honda Civic at a check point in tambon Yo Nok of Chiang Saen district, Chiang Rai, on Tuesday, Pol Gen Srivara said.
Suspect Wichan had ordered the ammunition. He was arrested at his house in tambon Tha Luang of Tha Rua district, Ayutthaya, in possession of 9mm and M16 ammunition.
Then police learned that some guns and bullets were hidden at the houses of Ms Prakongsri in Sao Hai district of Saraburi and of Mr Winas in Ban Mo district of the same province.
There police seized five M26 grenades, some pistols, about 4,500 5.56mm bullets and 50 9mm and .38 bullets.
Some weapons had been sold to Mr Praditthong and Mr Sombat. They were arrested and many more guns and bullets were seized at their houses in Chiang Rai and Chachoengsao provinces.
Mr Wichan allegedly told police that in the past 3-4 months he sold weapons to a middleman who communicated through the Line chat application.
Pol Gen Srivara said the suspects were connected to hardcore red-shirt political activists and would supply weapons to create a situation during the pro-election rally.
The seized weapons were only one-third of their weapons cache, he said.
The deputy police chief also quoted bomb experts as saying that the grenades seized from the suspects shared some serial numbers with those used in about 15 violent attacks during the red-shirt political protests in 2010.
Police were watching for hardcore red-shirt members at the Imperial Lat Phrao shopping centre and Thammasat University's Tha Phra Chan campus in Bangkok, Pol Gen Srivara said.
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