Andrie Yunus, an outspoken critic of the military’s expanding power in Indonesia, was riding his motorbike last month in Jakarta when two men rode past and threw acid in his face. He suffered burns to 24 percent of his body and damage to his right eye.

The ambush, which recalled Indonesia’s decades under a military dictatorship, was captured on street surveillance cameras. Within days, the authorities arrested four members of a military intelligence unit — a captain, two lieutenants and a sergeant. Their trial in a military court began Wednesday.

But human rights activists say that many more people were involved in the attack, and that the rush to try the four men is part of an attempt to shield the mastermind who authorized it.

“This was an intelligence operation,” said Fadhil Alfathan, a member of the Advocacy Team for Democracy, a coalition of rights groups backing Mr. Andrie. “It was a well-planned and well-trained operation, starting with surveillance, stalking, then tailing, execution and escape.”

The assault on Mr. Andrie, 27, the deputy coordinator for the prominent rights group, Kontras, is reminiscent of the brutality of the 32-year dictatorship under Suharto, who stepped down in 1998 in the face of massive pro-democracy protests. Now rights activists fear that the military’s influence is growing again under the current president, Prabowo Subianto, Suharto’s former son-in-law and once a widely feared general.

Mr. Andrie’s supporters say the acid attack on the evening of March 12 was a premeditated attempt to kill him because of his criticism of the military’s increasing role in government, including a law adopted after Mr. Prabowo took office that allows active armed forces officers to hold more government posts.

More than 420 rights groups and hundreds of activists around the world have signed a statement condemning the attack and calling for it to be investigated “thoroughly and transparently.”

The police conducted an initial investigation of the attack and quickly identified two suspects who belonged to the military.

The Advocacy Team for Democracy obtained and analyzed street surveillance videos from the weeks before the assault and concluded that at least 16 operatives were involved in following and surveilling Mr. Andrie.

Mr. Prabowo promised a thorough investigation.

“This is a barbaric act, we must pursue it,” the president said in remarks released a week after the attack. “We must investigate. Who ordered them, who paid.”

Days later, the general who headed the military’s main intelligence arm, the Strategic Intelligence Agency, quietly resigned. He is not known to be facing any charges.

But on March 31, the police announced that the case had been transferred to the military, which meant that military prosecutors would have the ability to limit the scope of the investigation and determine what information is made public.

Mr. Andrie came to public attention in March of last year when he led a small group of protesters who barged into a closed parliamentary meeting at a luxury hotel in Jakarta. The lawmakers were discussing the measure to let active military officers hold government posts.

In a scene captured on video, Mr. Andrie railed against the legislation before security officers pushed the protesters from the room.

The measure was later approved by Parliament and signed into law by Mr. Prabowo.

In a letter this month to Constitutional Court judges who are reviewing the law,Mr. Andrie objected to the transfer of the assault case to a military court, saying such courts have been “a hotbed of impunity for soldiers who commit human rights violations.”

“In this case, if it is not tried in the public court,” he added, “it is a serious violation of the principle of equality before the law.”

The military prosecutor handling the case announced in mid-April that the attack by the four operatives was motivated by a “personal vendetta” against Mr. Andrie.

The suspects are charged with assault causing serious injury and premeditated assault. They face a maximum of 12 years in prison.

Mr. Andrie has been hospitalized since the attack. He has not been interviewed by anyone from the police or the military, said Lakso Anindito, a lawyer from the Advocacy Team for Democracy who is representing him.

Mr. Lakso said he does not expect that his client will be called to testify.

He said he believes the attackers moved to silence him a year after the hotel protest because he was relentless in his criticism of the military and the 2025 law.

“It’s because Andrie never stops,” he said. “He is persistent in fighting this law. And an attack like this doesn’t just happen. It takes at least two months for them before it leads to that day.”

The attackers were so bold that they rode up to him and threw acid in his face despite the visible presence of numerous surveillance cameras in the area.

One video that captures Mr. Andrie seconds after the ambush shows him jumping off his motorbike, ripping off his shirt and screaming in agony as a crowd quickly gathers. Doctors say he suffered severe chemical burns on his face, hands, neck and chest. His helmet and visor saved him from even more serious harm.

Doctors have operated on Mr. Andrie five times in an effort to save the sight in his right eye.

In a letter to the president, Mr. Andrie called on him to ensure that the trial of his accused attackers adheres to the principle of due process, free from “corrupt interests.”

“This case is not solely about me,” he concluded, “but about the state’s commitment to protecting its citizens and upholding the law fairly.”

Indonesia has a history of impunity in the handling of attacks on human rights activists and corruption fighters.

The assault on Mr. Andrie is similar to a 2017 acid attack against a leading anti-corruption investigator, Novel Baswedan. Two police officers were found guilty of splashing sulfuric acid in Mr. Novel’s face as they rode by on a motorbike. The attack left him blind in one eye and half-blind in the other.

Mr. Novel, a senior investigator for Indonesia’s respected Corruption Eradication Commission, has said he was never able to determine who was behind the attack but believes it was someone he investigated. The police officers were sentenced to 18 and 24 months.

Mr. Andrie’s case also recalled the 2004 murder of Munir Said Thalib, the Kontras founder. Mr. Munir, a prominent critic of the military, was poisoned with arsenic while flying from Jakarta to Amsterdam and died before the plane landed. An off-duty Garuda airline pilot was found guilty of planning the murder but allegations that the state intelligence agency was behind the killing were never fully investigated.