BANGKOK (Reuters) - Unknown assailants attacked and wounded a Thai pro-democracy activist on Friday, apparently the latest victim of what rights groups call a systematic campaign of violence against critics of the army following a disputed election.

FILE PHOTO: Student activist Sirawith Seritiwat, an anti-coup activist, is detained by the police while demonstrating alone against a military-backed draft constitution in Bangkok, Thailand, May 1, 2016. REUTERS/Jorge Silva/File Photo

Sirawith Seritiwat, 27, was attacked by four men wielding clubs near his home in a Bangkok suburb leaving him in hospital with serious head injuries, his mother said.

“The doctor says they are concerned about his eyes because of the impact on the optic nerve,” Sirawith’s mother, Patnaraee Charnkit, told reporters.

“His eyes are open but he is unresponsive,” she said.

A police officer confirmed the late morning attack, saying the number of assailants was not known and police were investigating.

Rights groups say there have been seven violent attacks on activists since a March 24 election that resulted in former junta leader Prayuth Chan-ocha staying on as a civilian prime minister under election rules written by the junta.

A similar number of attacks on activists took place in the year before the vote, they say. Patnaraee said it was the second time this month that her son had been assaulted.

“What we’re seeing is a pattern,” Sunai Phasuk, senior Thai researcher for Human Rights Watch, said of the attacks on critics of the military’s involvement in politics.

“It shows that after the election, Thailand remains in a climate of fear and the country is not on a path toward a return to democratic rule.”

The military seized power in a 2014 coup and remains in charge until a new cabinet is sworn in, something that Prayuth says will likely happed by mid-July.

Opposition parties say the electoral system was designed to extend and legitimize military domination of civilian government.

A government spokesman was not immediately available for comment.