Pol Maj Gen Thanet Sunthornsuk, chief of the provincial police, declared the road open from noon Friday after protests on Thursday hampered traffic to southern provinces, forcing drivers to use alternative, local roads in Ban Sang and neighbouring Bang Saphan Noi district.
Protesters at the rally called on government to guarantee better rubber prices for farmers in the province, who say they are struggling to cope on their low income, given rising living costs.
Violence erupted on Thursday night and in the early hours of Friday when police decided to fire tear gas at the demonstrators and were countered by giant firecrackers and home-made ping-pong bombs.
Two vehicles were torched at 1am on Friday, before the violence ended at 2am.
The clash injured 25 policemen, one seriously. Reports said five farmers were hurt.
Police detained 12 protest leaders at Phramongkutklao Border Patrol Police camp for questioning.
Police said they suspect a group of youths was behind the clash.
Pol Maj Gen Thanet said some had come from other provinces to incite violence, adding that police were collecting evidence to ask the provincial court to issue arrest warrants for a number of young suspects.
Forensics experts meanwhile arrived at the scene of the violence to gather evidence, where farmers showed them tear gas cartridges fired by the police.
Santad Dejkerd, a protest leader in Bang Saphan, warned that bigger protests could be imminent after the clashes on Thursday night.
He urged the government to take responsibility for the fact some demonstrators were injured and called for fair investigations into the incident and proof to substantiate the claim that a group of youths from outside the province were partly to blame.
Mr Santad said farmers would seek bail for protest leaders who had been detained, and criticised the police for using excessive force to break up the protest.
Protesters will ask the Lawyers Council of Thailand to help them take legal against the police, he added, including some officers who allegedly sexually harassed female demonstrators.
The national rubber policy committee on Thursday stood by its Aug 14 resolution on measures to help rubber growers through a subsidy of 1,260 baht per rai, although it agreed to extend the cap on the subsidy from 10 to 25 rai per farmer.
But farmers want guaranteed prices instead, and have been demanding at least 100 baht per kilogramme for unsmoked rubber sheet.
A meeting of the confederation of rubber farmers in Surat Thani agreed to lower the asking price to a guaranteed 92 baht per kilogramme, and urged for the end of rallies in all locations.
A meeting between southern farmers and a government committee led by Deputy Prime Minister Pol Gen Pracha Promnok was meanwhile held in Nakhon Si Thammarat, in a bid to thrash out differences between the sides.