The two men came from behind as Worachet Pakeerut parked his car. One punched him in the face and the pair sped off on a motorbike.
"It happened so fast I couldn't see their faces," the law professor and activist said of the attack, which left him bleeding and needing hospital treatment. "Fortunately, other people could see them and were able to describe them to the police."
Days afterwards, the men - brothers - turned themselves in. They told police they attacked the professor because of his campaign to change a strict law relating to the Thai monarchy.
The attack on Worachet underscores the increasingly bitter nature of a struggle over Thailand's controversial "lese majeste" law, which outlaws criticism of the royal family, in a battle that threatens to reopen old political wounds.
Campaigners such as Worachet say the law is unfair, unaccountable and has increasingly been used against political targets and to quieten dissidents.
But groups who want to retain it, and have taken to the streets to demonstrate, say it is important for retaining harmony with the country. They say only those who intended to insult the monarchy are charged.
At present, someone can be jailed for up to 15 years if convicted of lese majeste, which is part of the legal code relating to national security. Anyone can make an allegation. Plans to alter article 112 of the criminal code would reduce the maximum jail term to three years and scrap the minimum sentence.
One campaigner, Puangthong Pawakapan, a professor of political science at Chulalongkorn University, said that it was almost impossible to debate the issue openly. "In Thailand, the King is seen as sacred and [people] believe he is beyond criticism," she said. "I think people outside of Thailand know more about the problems of this law. Also, the Thai mainstream media ignores this issue."
Those who monitor lese majeste say there are now dozens of cases every year, compared with just a handful two decades ago. Many prosecutions relate to internet postings.
When Prime Minister Yingluck Shinawatra was elected last year, she vowed that her Government would review the lese majeste law. That has not happened, apparently because of protests and the lingering fear that the Government could be ousted.